Display device and electronic device including display device

ABSTRACT

Objects are to provide a display device the power consumption of which is reduced, to provide a self-luminous display device the power consumption of which is reduced and which is capable of long-term use in a dark place. A circuit is formed using a thin film transistor in which a highly-purified oxide semiconductor is used and a pixel can keep a certain state (a state in which a video signal has been written). As a result, even in the case of displaying a still image, stable operation is easily performed. In addition, an operation interval of a driver circuit can be extended, which results in a reduction in power consumption of a display device. Moreover, a light-storing material is used in a pixel portion of a self-luminous display device to store light, whereby the display device can be used in a dark place for a long time.

TECHNICAL FIELD

The present invention relates to display devices and electronic deviceseach including the display device. In particular, one embodiment of thepresent invention relates to a display device including a field effecttransistor in which an oxide semiconductor is used and a light-emittingelement, and an electronic device including the display device.

BACKGROUND ART

A technique for forming a thin film transistor (TFT) by using asemiconductor thin film formed over a substrate having an insulatingsurface has attracted attention. The thin film transistor has been usedin a display device typified by a liquid crystal television. Asilicon-based semiconductor material is known as a material of asemiconductor thin film that can be used in a thin film transistor.Besides, an oxide semiconductor has attracted attention as anothermaterial.

As a material of the oxide semiconductor, zinc oxide or a substancecontaining zinc oxide is known. Thin film transistors each of which isformed using an amorphous oxide (an oxide semiconductor) having anelectron carrier concentration of lower than 10¹⁸/cm³ are disclosed inPatent Documents 1 to 3.

A field-effect transistor in which an oxide semiconductor is used can beused in, for example, a display device. As examples of the displaydevice, the following are given: a self-luminous display device fordisplaying an image by emitting light, a transmissive display device fordisplaying an image by selectively transmitting part of light from abacklight, and a reflective display device for displaying an image byreflecting external light.

The self-luminous display device and the transmissive display device areinsusceptible to external light, emit bright light, and have excellentcapability to display images.

The reflective display device has a feature in that power saving iseasily achieved because it does not need to incorporate a light source.Needless to say, displayed images can be rewritten. Therefore, thereflective display device has attracted attention as electronic papersubstituted for a print medium from societies aiming for saving ofnatural resources.

However, the visibility of the reflective display device is decreased inan environment where the amount of external light is small; thus,lighting is needed for using the reflective display device. The use oflighting damages the feature of the reflective display device of lowpower consumption. Hence, inventions, in each of which a substance thatstores external light such as a light-storing fluorescent material or alight-storing material is used in a reflective display device forincreasing the visibility in a dark place with power consumptionsuppressed, are disclosed in Patent Documents 4 and 5.

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.    2006-165527-   [Patent Document 2] Japanese Published Patent Application No.    2006-165528-   [Patent Document 3] Japanese Published Patent Application No.    2006-165529-   [Patent Document 4] Japanese Published Patent Application No.    2006-3924-   [Patent Document 5] Japanese Published Patent Application No.    2008-116855

DISCLOSURE OF INVENTION

Deviation from the stoichiometric composition of an oxide semiconductorarises when the oxide semiconductor is formed as a thin film. Forexample, the electric conductivity of the oxide semiconductor is changeddue to excess or deficiency of oxygen. In addition, hydrogen which ismixed into the oxide semiconductor during the thin film formation formsan oxygen (O)-hydrogen (H) bond and the OH bond serves as an electrondonor, which is a factor for changing the electric conductivity.Furthermore, O—H is a polar molecule, and thus serves as a factor forchanging characteristics of an active device such as a thin filmtransistor manufactured using the oxide semiconductor.

Even when the electron carrier concentration is lower than 10¹⁸/cm³, theoxide semiconductor is substantially n-type, and the on/off ratio of thethin film transistors disclosed in the above patent documents is only10³. The above low on/off ratio of the thin film transistors is causedby large off-state current.

In the case where a thin film transistor with a large amount ofoff-state current is used in a pixel portion of a display device, acapacitor needs to be additionally provided for keeping signal voltageapplied to a pixel. The provision of the capacitor in the pixel causesproblems such as a decrease in aperture ratio of the pixel and anincrease in power consumption of the display device.

Further, a decrease in energy supplied to a light-emitting elementincluded in a self-luminous display device or a transmissive displaydevice for reducing power consumption of the display device has asignificant influence on the display quality such as dark display ordisplay disappearance.

Further, the reflective display devices in each of which a light-storingmaterial is used, which are disclosed in the above patent documents, canperform display with low power consumption even in an environment wherethe amount of external light is small; however, the light-storingmaterial needs to be irradiated with external light in advance in orderto store the light. Therefore, such a reflective display device is notsuitable for long-term use in a dark place.

The present invention is made in view the foregoing technicalbackground. Therefore, an object of one embodiment of the presentinvention is to provide a display device the power consumption of whichis suppressed. Another object is to provide a self-luminous displaydevice the power consumption of which is suppressed. Another object isto provide a self-luminous display device the power consumption of whichis suppressed and which is capable of long-term use in a dark place.

One embodiment of the present invention is a self-luminous displaydevice in which a thin film transistor having stable electriccharacteristics (e.g., an extremely small amount of off-state current)is used. Specifically, in the self-luminous display device, a drivercircuit of a light-emitting element includes a thin film transistor inwhich a channel region is formed using an oxide semiconductor which isan intrinsic semiconductor or a substantially intrinsic semiconductor byremoval of impurities that form as an electron donor (a donor) in theoxide semiconductor and which has a larger energy gap than a siliconsemiconductor.

In other words, in one embodiment of the present invention, a drivercircuit of a light-emitting element includes a thin film transistor inwhich a channel formation region is formed using an oxide semiconductorfilm in which hydrogen or OH groups contained in the oxide semiconductoris reduced to a concentration of hydrogen of 5×10¹⁹/cm³ or lower,preferably 5×10¹⁸/cm³ or lower, more preferably 5×10¹⁷/cm³ or lower andin which the carrier concentration is lower than 1×10¹⁴/cm³, preferably1×10¹²/cm³ or lower.

The energy gap is 2 eV or more, preferably 2.5 eV or more, morepreferably 3 eV or more, impurities such as hydrogen that forms a donorare reduced as much as possible, and the carrier concentration is lowerthan 1×10¹⁴/cm³, preferably 1×10¹²/cm³ or lower.

The oxide semiconductor that is highly purified as described above isused in a channel formation region of a thin film transistor, wherebythe thin film transistor functions so that the drain current is 1×10⁻¹³A or less in a range of a gate voltage of −5 V to −20 V when the drainvoltage is 1 V and 10 V, even in the case where the channel width is 10mm.

Further, one embodiment of the present invention focuses on powerconsumed by a driver circuit included in a self-luminous display device.In other words, power consumed by the display device may be reduced bydecreasing the frequency of operation of the driver circuit. Inaddition, power consumed by the display device may be reduced in such amanner that a light-storing layer is provided in a pixel portion of theself-luminous display device, light emitted from a light-emittingelement is stored in the light-storing layer, and energy supplied to thelight-emitting element is suppressed while an image is displayed withthe use of light emitted from the light-storing layer.

In other words, one embodiment of the present invention is a displaydevice including, in a pixel, a power supply line to which pulsed DCpower is supplied, a light-emitting element to which power is suppliedfrom the power supply line, a first thin film transistor which controlsswitching of a circuit which connects the power supply line to thelight-emitting element, a signal line which supplies a video signal, anda second thin film transistor which controls switching of a circuitwhich connects the signal line to the first thin film transistor. Achannel formation region in the second thin film transistor is formedusing an oxide semiconductor which has a band gap of 2 eV or more and ahydrogen concentration of 5×10¹⁹/cm³ or lower. The second thin filmtransistor in which off-state current per a channel width of 1 μm issuppressed to 1×10⁻¹⁶ A /μm or lower holds the first thin filmtransistor in an on state and connects the power supply line to thelight-emitting element to display a still image.

One embodiment of the present invention is the display device which hasa concentration of carriers in an oxide semiconductor layer of lowerthan 1×10¹⁴/cm³.

One embodiment of the present invention is the display device having aperiod in which output of a scan line signal is stopped, in a period inwhich a still image is displayed.

One embodiment of the present invention is the display device includinga light-emitting element including a pair of electrodes and a layercontaining a light-emitting organic substance between the pair ofelectrodes.

One embodiment of the present invention is the display device includinga light-storing layer in a pixel.

One embodiment of the present invention is an electronic deviceincluding the display device.

Note that in this specification, a “light-storing material” refers tomaterials in general which absorbs external energy such as externallight, is relatively stable, and generates an exciton with a long lifeand in which the exciton is deactivated after light emission for arelatively long time. The light-storing material in which the excitonwith a long life is stored continues to emit light even when there is noexternal energy.

Note that in this specification, a “pixel” refers to an element groupwhich includes elements provided in each pixel of a display device, forexample, an element for controlling display in accordance with electricsignals, such as a thin film transistor, an electrode functioning as apixel electrode, or a wiring. Note that a pixel may include a colorfilter or the like, and may correspond to one color component whoseilluminance can be controlled by one pixel. Thus, for example, in thecase of a color display device including color components of R, G, andB, a minimum unit of an image includes three pixels of R, G, and B andan image can be obtained by a plurality of pixels.

Note that in this specification, when it is described that “A and B areconnected”, the case where A and B are electrically connected, the casewhere A and B are functionally connected, and the case where A and B aredirectly connected are included therein. Here, each of A and Bcorresponds to an object (e.g., a device, an element, a circuit, awiring, an electrode, a terminal, a conductive film, or a layer).

Note that in this specification, an “EL layer” refers to a layerprovided between a pair of electrodes in a light-emitting element. Thus,a light-emitting layer containing an organic compound that is alight-emitting substance which is interposed between electrodes is anembodiment of the EL layer.

In this specification, in the case where a substance A is dispersed in amatrix formed using a substance B, the substance B forming the matrix isreferred to as a host material, and the substance A dispersed in thematrix is referred to as a guest material. Note that the substance A andthe substance B may each be a single substance or a mixture of two ormore kinds of substances.

Note that in this specification, a “light emitting device” refers to animage display device, a light emitting device, or a light source(including a lighting device). In addition, the light-emitting deviceincludes any of the following modules in its category: a module in whicha connector such as an FPC (flexible printed circuit), a TAB (tapeautomated bonding) tape, or a TCP (tape carrier package) is attached toa light-emitting device; a module having a TAB tape or a TCP providedwith a printed wiring board at the end thereof; and a module having anIC (integrated circuit) directly mounted over a substrate over which alight-emitting element is formed by a COG (chip on glass) method.

According to one embodiment of the present invention, a circuit isformed using a thin film transistor in which a highly-purified oxidesemiconductor is used, whereby operation of the circuit included in adisplay device can be stabilized. In addition, off-state current reducedto 1×10⁻¹³ A or lower allows omission of a capacitor which holds signalvoltage applied to a pixel of the display device. In other words, astorage capacitor does not need to be provided in each pixel, whichresults in an increase in aperture ratio. When aperture ratio isincreased, driving voltage of a light-emitting element is suppressed,which results in a reduction in power consumption of the display device.

Further, a pixel which is formed using the thin film transistoraccording to one embodiment of the present invention can maintain aconstant state (a state in which an image signal is written), and thuscan operate stably even in the case where a still image is displayed. Inaddition, operation interval of a driver circuit can be extended, whichresults in a reduction in power consumption of a display device.

Further, according to one embodiment of the present invention, a displaydevice that can be used in an environment where external light is weakcan be provided. In addition, a display device that can display an imagewith power consumption suppressed can be provided.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIG. 1A is a top view of a pixel according to Embodiment and FIGS. 1Band 1C are cross-sectional views thereof;

FIG. 2 illustrates a structure of a display device according toEmbodiment;

FIG. 3 shows a period of writing to a pixel and a retention periodaccording to Embodiment;

FIG. 4A is a top view of a pixel according to Embodiment and FIG. 4B isa cross-sectional view thereof;

FIG. 5A is a top view of a thin film transistor according to Embodimentand FIG. 5B is a cross-sectional view thereof;

FIGS. 6A to 6E illustrate manufacturing steps of a thin film transistoraccording to Embodiment;

FIG. 7A is a top view of a thin film transistor according to Embodimentand FIG. 7B is a cross-sectional view thereof;

FIGS. 8A to 8E illustrate manufacturing steps of a thin film transistoraccording to Embodiment;

FIGS. 9A and 9B are cross-sectional views of thin film transistorsaccording to Embodiment;

FIGS. 10A to 10E illustrate manufacturing steps of a thin filmtransistor according to Embodiment;

FIGS. 11A to 11E illustrate manufacturing steps of a thin filmtransistor according to Embodiment;

FIGS. 12A to 12D illustrate manufacturing steps of a thin filmtransistor according to Embodiment;

FIGS. 13A to 13D illustrate manufacturing steps of a thin filmtransistor according to Embodiment;

FIG. 14 is a cross-sectional view of a thin film transistor according toEmbodiment;

FIG. 15 is a diagram illustrating an equivalent circuit of a pixelaccording to Embodiment;

FIGS. 16A to 16C are cross-sectional views of pixels according toEmbodiment;

FIG. 17A is a top view of a light-emitting device according toEmbodiment and FIG. 17B is a cross-sectional view thereof;

FIG. 18 is a cross-sectional view of a pixel according to Embodiment;

FIGS. 19A to 19C illustrate electronic devices according to Embodiment;

FIGS. 20A to 20C illustrate electronic devices according to Embodiment;

FIG. 21 is a longitudinal sectional view of an inverted staggered thinfilm transistor in which an oxide semiconductor is used;

FIGS. 22A and 22B are energy band diagrams (schematic views) in a crosssection taken along line A-A′ in FIG. 21

FIG. 23A shows a state in which positive potential (+V_(G)) is appliedto a gate (G1) and FIG. 23B shows a state in which negative potential(−V_(G)) is applied to a gate (G1);

FIG. 24 shows a relation between a vacuum level and a work function(ϕ_(M)) of a metal and a relation between a vacuum level and an electronaffinity (χ) of an oxide semiconductor;

FIG. 25 is a block diagram illustrating a structure of a display deviceaccording to Embodiment;

FIGS. 26A to 26C illustrate structures of a driver circuit and a shiftregister according to Embodiment;

FIG. 27 is a timing chart of an operation of a shift register; and

FIG. 28 is a timing chart of an operation of a shift register.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, Embodiments and Example of the present invention will bedescribed in detail with reference to the accompanying drawings.However, the present invention is not limited to the description below,and it is easily understood by those skilled in the art that modes anddetails disclosed herein can be modified in various ways withoutdeparting from the spirit and scope of the present invention. Therefore,the present invention is not construed as being limited to thedescription of Embodiments. In the structures to be given below, thesame portions or portions having similar functions are denoted by thesame reference numerals in different drawings, and explanation thereofwill not be repeated.

(Embodiment 1)

In this embodiment, an example of a pixel including a thin filmtransistor in which a highly-purified oxide semiconductor is used and apixel electrode will be described below with reference to FIGS. 1A to1C, FIG. 2, FIG. 3, and FIGS. 4A and 4B.

First, FIG. 1A is a top view of the pixel. Note that as an example of astructure of a TFT illustrated in FIG. 1A, a bottom gate structure thatis a so-called inverted staggered structure is illustrated, in whichwiring layers that serve as source and drain electrodes of a TFT areprovided on one side of an oxide semiconductor layer that serves as achannel region, and a wiring that serves as a gate is provided on theother side of the oxide semiconductor layer. A pixel 100 illustrated inFIG. 1A includes a first wiring 101 that functions as a scan line, asecond wiring 102A that functions as a signal line, an oxidesemiconductor layer 103, a capacitor line 104, and a pixel electrode105, and in addition to the above, a third wiring 102B for electricallyconnecting the oxide semiconductor layer 103 to the pixel electrode 105,whereby a thin film transistor 106 is formed.

The first wiring 101 also functions as a gate of the thin filmtransistor 106. The second wiring 102A also functions as one of thesource electrode and the drain electrode and one electrode of a storagecapacitor. The third wiring 102B also functions as the other of thesource electrode and the drain electrode. The capacitor line 104functions as the other electrode of the storage capacitor. Note that thefirst wiring 101 and the capacitor line 104 are formed in the samelayer, and the second wiring 102A and the third wiring 102B are formedin the same layer. In addition, the third wiring 102B and the capacitorline 104 partly overlap with each other to form a storage capacitor of alight-emitting element. Note that the oxide semiconductor layer 103included in the thin film transistor 106 is provided over a wiringbranching from the first wiring 101 with a gate insulating film (notillustrated) between the oxide semiconductor layer 103 and the wiring.

FIG. 1B illustrates a cross-sectional structure taken along chain lineA1-A2 in FIG. 1A. In the cross-sectional structure illustrated in FIG.1B, the first wiring 101 that functions as a gate and the capacitor line104 are provided over a substrate 111 with a base film 112 therebetween.A gate insulating film 113 is provided so as to cover the first wiring101 and the capacitor line 104. The oxide semiconductor layer 103 isprovided over the gate insulating film 113. The second wiring 102A andthe third wiring 102B are provided over the oxide semiconductor layer103. An oxide insulating layer 114 that functions as a passivation filmis provided over the oxide semiconductor layer 103, the second wiring102A, and the third wiring 102B. An opening is formed in the oxideinsulating layer 114, and the pixel electrode 105 and the third wiring102B are connected to each other in the opening. A capacitor isconstituted by the third wiring 102B, the capacitor line 104, and thegate insulating film 113 used as a dielectric.

FIG. 1C is a cross-sectional view taken along chain line B1-B2 in FIG.1A and illustrates a structure in which an insulating layer 121 isprovided between the capacitor line 104 and the second wiring 102A.

In the case where the second wiring 102A is provided over the firstwiring 101 and the capacitor line 104, parasitic capacitance might begenerated between the first wiring 101 and the second wiring 102A andbetween the capacitor line 104 and the second wiring 102A, depending onthe thickness of the gate insulating film 113. For that reason, theinsulating layer 121 is provided as illustrated in FIG. 1C, so thatdefects such as a malfunction can be reduced.

Note that the pixel illustrated in FIGS. 1A to 1C corresponds to one ofa plurality of pixels 201 arranged in matrix over a substrate 200 asillustrated in FIG. 2. FIG. 2 illustrates a structure in which a pixelportion 202, a scan line driver circuit 203, and a signal line drivercircuit 204 are provided over the substrate 200. Whether the pixels 201are in a selected state or in a non-selected state is determined per rowin accordance with a scan signal supplied from the first wiring 101connected to the scan line driver circuit 203. The pixel 201 selected bythe scan signal is supplied with video voltage (also referred to as avideo signal, an image signal, or video data) from the second wiring102A connected to the signal line driving circuit 204.

Although the scan line driver circuit 203 and the signal line drivercircuit 204 are provided over the substrate 200 in FIG. 2, either one ofthe scan line driver circuit 203 and the signal line driver circuit 204may be provided over the substrate 200. Alternatively, only the pixelportion 202 may be provided over the substrate 200.

FIG. 2 illustrates an example in which the plurality of pixels 201 arearranged in matrix (in stripe) in the pixel portion 202. Note that thepixels 201 are not necessarily arranged in matrix and may be arranged ina delta pattern or in a Bayer pattern. As a display method of thedisplay portion 202, a progressive method or an interlace method can beemployed. Note that color elements controlled in the pixel for colordisplay are not limited to three colors of R (red), G (green), and B(blue), and color elements of more than three colors may be employed,for example, RGBW (W corresponds to white), or RGB added with one ormore of yellow, cyan, magenta, and the like. Note that the sizes ofdisplay regions may be different between respective dots of colorelements.

In FIG. 2, the numbers of first wirings 101 and second wirings 102Acorrespond to the number of pixels in the row and column directions. Thepixel 201 may be driven by the first wiring 101 and the second wiring102A which are shared by plural pixels.

Note that, although the second wiring 102A is rectangular in FIG. 1A,the second wiring 102A may surround the third wiring 102B (specifically,the second wiring 102A may be U-shaped or C-shaped) so that an area of aregion in which carriers move is increased to increase the amount ofcurrent flowing when the thin film transistor is in a conducting state(the current is also referred to as on-state current).

Note that “on-state current” described in this specification refers tocurrent that flows between a source and a drain when a thin filmtransistor is on (when a thin film transistor is in a conducting state).In the case of an n-channel thin film transistor, “on-state current”refers to current that flows between a source and a drain when voltageapplied between a gate and the source is higher than the thresholdvoltage (V_(th)).

Note that aperture ratio refers to a ratio of an area of a regionthrough which light is transmitted per pixel; the aperture ratio isdecreased, as an area of a region occupied by components that do nottransmit light is increased, whereas the aperture ratio is increased asan area of a region occupied by components that transmit light isincreased. In a display device, the aperture ratio is increased byreduction in the area occupied by a wiring and a capacitor line thatoverlap with a pixel electrode and reduction in the size of a thin filmtransistor.

In particular, in a self-luminous display device, aperture ratio refersto proportion of an emission area of a light-emitting element that canbe observed by an observant in front of a display of the display devicein an area of pixels.

Note that a thin film transistor is an element having at least threeterminals of a gate, a drain, and a source. The thin film transistor hasa channel region between a drain region and a source region, and currentcan flow through the drain region, the channel region, and the sourceregion. Here, since the source and drain of the transistor might bechanged depending on the structure, the operating condition, and thelike of the transistor, it is difficult to define which is a source or adrain. Therefore, a region functioning as source and drain is not calledthe source or the drain in some cases. In such a case, for example, oneof the source and the drain may be referred to as a first terminal andthe other thereof may be referred to as a second terminal.Alternatively, one of the source and the drain may be referred to as afirst electrode and the other thereof may be referred to as a secondelectrode. Further alternatively, one of the source and the drain may bereferred to as a first region and the other thereof may be called asecond region.

Next, the oxide semiconductor layer 103 will be described.

Hydrogen or OH groups are removed from an oxide semiconductor used inthis embodiment, and the concentration of hydrogen contained in theoxide semiconductor is 5×10¹⁹/cm³ or lower, preferably 5×10¹⁸/cm³ orlower, more preferably 5×10¹⁷/cm³ or lower. A channel formation regionis formed in an oxide semiconductor film the carrier concentration ofwhich is lower than 1×10¹⁴/cm³, preferably 1×10¹²/cm³ or lower so that athin film transistor is formed. The concentration of hydrogen in theoxide semiconductor film may be measured by secondary ion massspectrometry (SIMS).

The energy gap of the oxide semiconductor is 2 eV or more, preferably2.5 eV or more, more preferably 3 eV or more, impurities such ashydrogen that forms a donor are reduced as much as possible so that thecarrier concentration is lower than 1×10¹⁴/cm³, preferably 1×10¹²/cm³ orlower. In other words, the carrier concentration of the oxidesemiconductor layer is reduced to a level as close to zero as possible.

The oxide semiconductor layer which is highly-purified by drasticallyremoving hydrogen contained in the oxide semiconductor layer asdescribed above is used in a channel formation region of a thin filmtransistor, whereby a thin film transistor with an extremely smallamount of off-state current can be provided.

For example, even in the case where a thin film transistor in which thehighly-purified oxide semiconductor layer is used has a channel lengthof 3 μm and a channel width of 10 mm, the thin film transistor functionsso that the drain current is 1×10⁻¹³ A or less in a range of a gatevoltage of −5 V to −20 V (the thin film transistor is off), when thedrain voltage is 1 V and 10 V.

Characteristics of a thin film transistor in which a highly-purifiedoxide semiconductor layer is used are described with reference to FIG.21, FIGS. 22A and 22B, FIGS. 23A and 23B, and FIG. 24. FIG. 21 is alongitudinal sectional view of an inverted staggered thin filmtransistor in which an oxide semiconductor is used. An oxidesemiconductor layer (OS) is provided over a gate electrode (GEl) with agate insulating film (GI) therebetween, and a source electrode (S) and adrain electrode (D) are provided thereover.

FIGS. 22A and 22B are energy band diagrams (schematic diagrams) of thethin film transistor in cross section along line A-A′ in FIG. 21. FIG.22A shows a case where the source and the drain have voltage of the samepotential (V_(D)=0 V). FIG. 22B shows a case where positive potential isapplied to the drain (V_(D)>0 V) whereas positive potential is notapplied to the source.

FIGS. 23A and 23B are energy band diagrams (schematic diagrams) of thethin film transistor in cross section along line B-B′ in FIG. 21. FIG.23A shows a state where positive potential (+V_(G)) is applied to a gate(G1), that is, a case where the thin film transistor is in an on statewhere carriers (electrons) flow between the source and the drain. FIG.23B shows a state where negative potential (−V_(G)) is applied to thegate (G1), that is, a case where the thin film transistor is in an offstate (where minority carriers do not flow).

FIG. 24 shows relation between the vacuum level and the work function(ϕ_(M)) of a metal and relation between the vacuum level and theelectron affinity (χ) of an oxide semiconductor.

A conventional oxide semiconductor is generally of n-type, and the Fermilevel (E_(F)) in that case is positioned closer to the conduction bandand is away from the intrinsic Fermi level (E_(i)) that is located inthe middle of the band gap. Note that it is known that some hydrogen inthe oxide semiconductor form a donor and might be a factor that causesan oxide semiconductor to be an n-type oxide semiconductor.

In contrast, the oxide semiconductor according to the present inventionis an oxide semiconductor that is made to be an intrinsic (i-type)semiconductor or made to be as close to an intrinsic semiconductor aspossible by being highly purified by removal of hydrogen that is ann-type impurity so that as few impurities, which are not main componentsof the oxide semiconductor, as possible are contained. In other words,the oxide semiconductor according to the present invention has a featurein that it is made to be an i-type (intrinsic) oxide semiconductor ormade to be close thereto by being highly purified by removal ofimpurities such as hydrogen or water as much as possible. As a result,the Fermi level (E_(F)) can be at substantially the same level as theintrinsic Fermi level (E_(i)).

It is said that the electron affinity (χ) of an oxide semiconductor is4.3 eV in the case where the band gap (Eg) thereof is 3.15 eV. The workfunction of titanium (Ti) used for forming the source and drainelectrodes is substantially equal to the electron affinity (χ) of theoxide semiconductor. In the case where titanium is used for the sourceand drain electrodes, the Schottky electron barrier is not formed at aninterface between the metal and the oxide semiconductor.

In other words, an energy band diagram (a schematic diagram) like FIG.22A is obtained in the case where a metal and an oxide semiconductor arein contact with each other when the work function (ϕ_(M)) of the metaland the electron affinity (χ) of the oxide semiconductor aresubstantially equal.

In FIG. 22B, a black circle (●) represents an electron. When positivepotential is applied to the drain, the electrons cross a barrier (h) tobe injected into the oxide semiconductor and flow toward the drain. Inthat case, the height of the barrier (h) changes depending on the gatevoltage and the drain voltage. In the case where positive drain voltageis applied, the height of the barrier (h) is smaller than the height ofthe barrier (h) in FIG. 22A of the case where no voltage is applied;that is, the height of the barrier (h) is smaller than half of the bandgap (Eg).

In this case, as shown in FIG. 23A, the electron moves along the lowestpart of the oxide semiconductor, which is energetically stable, at aninterface between the gate insulating film and the highly-purified oxidesemiconductor.

In FIG. 23B, when negative potential is applied to the gate (G1), thenumber of holes that are minority carriers is substantially zero; thus,the current value becomes a value as close to zero as possible.

For example, even when the thin film transistor has a channel width of1×10⁴ μm and a channel length of 3 μm, an off-state current of 10⁻¹³ Aor lower and a subthreshold value (S value) of 0.1 V/dec. (the thicknessof the gate insulating film: 100 nm) can be obtained.

As described above, the oxide semiconductor is highly purified so thatthe amount of impurities that are not main components of the oxidesemiconductor are minimized, whereby favorable operation of the thinfilm transistor can be obtained.

A memory circuit (a memory element) or the like that is manufacturedusing such a thin film transistor that has an extremely small currentvalue (an extremely small off-state current value) at the time when thethin film transistor is off has a small off-state current value andlittle leakage current. Thus, the length of a period for holding anelectric signal such as a video signal can be increased.

Specifically, the thin film transistor including the above oxidesemiconductor layer can have an off-state current per a channel width of1 μm of 1×10⁻¹⁶ A/μm or lower, furthermore 1 aA/μm (1×10⁻¹⁸ A/μm) orlower.

The length of the period for holding an electric signal such as a videosignal can be increased by using a transistor that has an extremelysmall current value (an off-state current value) at the time when thetransistor is off as a switching transistor in a pixel portion (e.g., aswitching transistor 6401 in FIG. 15). Since the length of the holdingperiod can be increased, for example, a writing interval can be longerthan or equal to 10 seconds, preferably longer than or equal to 30seconds, more preferably longer than or equal to 1 minute and shorterthan 10 minutes. Increase in the writing interval makes it possible toincrease an effect of suppressing power consumption.

Note that the suppression of off-state current in the transistor can bereferred as off resistivity. The off resistivity refers to resistivityof a channel formation region at the time when the transistor is off,and the off resistivity can be calculated from off-state current.

Specifically, when the values of off-state current and drain voltage areknown, it is possible to obtain resistivity (off resistivity R) at thetime when the transistor is off in accordance with Ohm's law. Inaddition, off resistivity ρ can be obtained in accordance with theformula ρ=RA/L (R: off resistivity), if a cross-section area A of achannel formation region and length L of the channel formation region(the length corresponds to a distance between the source and drainelectrodes) are known.

Here, the cross-section area A can be obtained in accordance with theformula A=dW (d: the thickness of the channel formation region, W: thechannel width). In addition, the length L of the channel formationregion is a channel length L. In such a manner, off resistivity can becalculated from off-state current.

The off resistivity of the transistor including the oxide semiconductorlayer of this embodiment is preferably greater than or equal to 1×10⁹Ω·m, more preferably greater than or equal to 1×10¹⁰ Ω·m.

The pixel configuration illustrated in FIG. 15 is one embodiment of thisembodiment. For example, a storage capacitor can be provided between agate of a driving transistor 6402 and a power supply line 6407. Thestorage capacitor may be constituted by a pair of electrodes and aninsulating layer as a dielectric interposed between the pair ofelectrodes. The size of the storage capacitor is set so that a chargecan be stored for a predetermined period in consideration of off-statecurrent of the switching transistor 6401 or the like.

For example, design or the like is performed while estimating the amountof the off-state current of a thin film transistor includinglow-temperature polysilicon at around 1×10⁻¹² A. Thus, the voltageholding period of a thin film transistor including an oxidesemiconductor can be approximately 10⁵ times as long as the voltageholding period of the thin film transistor including low-temperaturepolysilicon, when the holding capacitance is equal (approximately 0.1pF). Further, a transistor including amorphous silicon has an off-statecurrent per a channel width of 1 μm of greater than or equal to 1×10⁻¹³A/μm. Thus, the voltage holding period of a thin film transistorincluding a highly-purified oxide semiconductor can be approximately 10⁴times as long as the voltage holding period of the thin film transistorincluding amorphous silicon, when the holding capacitance is equal(approximately 0.1 pF).

For example, in general, display is performed at 60 frames per second(16 msec per one frame) in a pixel including a transistor in whichlow-temperature polysilicon is used. The same can be said for a case ofa still image for the reason that decrease in rate (increase in writinginterval) causes decrease in voltage of a pixel and a defect of display.In contrast, in the case where the transistor including theabove-described oxide semiconductor layer is used, the amount ofoff-state current is small; thus, the holding period of one signalwriting can be approximately 1600 seconds, that is, 10⁵ times as long asthat of the transistor including low-temperature polysilicon.

As described above, the display device of this embodiment is capable ofperforming display of a still image with a small number of times imagesignals are written. A long holding period allows the frequency ofsignal writing to be reduced especially in the case of performingdisplay of a still image. Thus, a reduction in power consumption can beachieved. For example, the number of times of writing into a pixelduring one still-image display period can be one or n. Note that n isgreater than or equal to 2 and less than or equal to 10³. Thus, areduction in power consumption of the display device can be achieved.

Moreover, in this embodiment, a driver circuit portion is operated sothat output of a signal supplied to a signal line or a scan line isstopped in performing display of a still image, whereby power consumedby the driver circuit as well as the pixel portion can be suppressed.

FIG. 3 shows relation between a writing period and a holding period(also referred to as one frame period). In FIG. 3, periods 251 and 252correspond to holding periods and periods 261 and 262 correspond towriting periods. The thin film transistor including the abovehighly-purified oxide semiconductor layer can have a long holding period(the period 251 and the period 252); thus, the frequency of signalwriting can be significantly decreased especially in the case ofperforming display of a still image. Thus, in display of a still image,which needs a small number of display switching, the number of writingof a signal to a pixel can be reduced, which results in a reduction inpower consumption.

Note that voltage applied to a gate of a driving transistor connected toan EL element attenuates while a still image is displayed; thus, refreshoperation may be performed as appropriate in consideration of theproportion of stored voltage applied to the gate of the drivingtransistor. For example, refresh operation may be performed when voltageis dropped to a predetermined level from the value (the initial value)of voltage immediately after a signal is written to the gate of thedriving transistor. The predetermined level of voltage is preferably setto a level at which flicker is not detected from the initial value.Specifically, in the case of an image is an object to be displayed,refresh operation (rewriting) is preferably performed every time thevoltage reaches a value lower than the initial value by 1.0%, preferably0.3%. In the case of characters are objects to be displayed, refreshoperation (rewriting) is preferably performed every time the voltagereaches a value lower than the initial value by 10%, preferably 3%.

FIG. 4A is a top view of a configuration of a pixel in the case where astorage capacitor is not formed and FIG. 4B is a cross-sectional viewthereof. The configuration illustrated in FIGS. 4A and 4B correspond toviews in which the capacitor line in FIGS. 1A and 1B is omitted. As isclear from the top view of FIG. 4A and the cross-sectional view of FIG.4B, an area occupied by the pixel electrode 105 can be increased, thatis, the aperture ratio can be increased by using a thin film transistorincluding an oxide semiconductor layer. In addition, as is clear fromthe cross-sectional view of FIG. 4B, by using the thin film transistorincluding an oxide semiconductor layer, the capacitor lines can bereduced, which leads to an increase in the area occupied by the pixelelectrode 105, that is, an increase in aperture ratio.

As in this embodiment, a thin film transistor in which a highly-purifiedoxide semiconductor layer is used and which has an extremely smalloff-state current value is used in a pixel portion, whereby a displaydevice that is capable of displaying an image (especially a movingimage) even without provision of a storage capacitor in the pixelportion. In addition, even in the case of providing a storage capacitor,a period for holding voltage in the storage capacitor can be increased;thus, a display device the power consumption of which in displaying astill image or the like is reduced can be provided. Furthermore, by anincrease in aperture ratio, a display device including a high-resolutiondisplay portion can be provided.

This embodiment can be implemented in combination with any of thestructures of the other embodiments as appropriate.

(Embodiment 2)

In this embodiment, an example of a thin film transistor that can beapplied to the display device disclosed in this specification will bedescribed. A thin film transistor 410 described in this embodiment canbe used as the thin film transistor 106 in Embodiment 1.

An embodiment of a thin film transistor of this embodiment and anembodiment of a method for manufacturing the thin film transistor willbe described with reference to FIGS. 5A and 5B and FIGS. 6A to 6E.

FIG. 5A illustrates an example of a top surface structure of a thin filmtransistor and FIG. 5B illustrates an example of a cross-sectionalstructure thereof The thin film transistor 410 illustrated in FIGS. 5Aand 5B is a top-gate thin film transistor.

FIG. 5A is a top view of the top-gate thin film transistor 410 and FIG.5B is a cross-sectional view taken along line C1-C2 in FIG. 5A.

The thin film transistor 410 includes, over a substrate 400 having aninsulating surface, an insulating layer 407, an oxide semiconductorlayer 412, a source or drain electrode layer 415 a, a source or drainelectrode layer 415 b, a gate insulating layer 402, and a gate electrodelayer 411. A wiring layer 414 a and a wiring layer 414 b are provided incontact with the source or drain electrode layer 415 a and the source ordrain electrode layer 415 b, respectively, so that they are electricallyconnected to each other.

Although the thin film transistor 410 is described as a single-gate thinfilm transistor, a multi-gate thin film transistor including a pluralityof channel formation regions can be formed when needed.

A process of manufacturing the thin film transistor 410 over thesubstrate 400 will be described below with reference to FIGS. 6A to 6E.

Although there is no particular limitation on a substrate which can beused as the substrate 400 having an insulating surface, it is necessarythat the substrate at least have heat resistance high enough towithstand heat treatment performed later. A glass substrate formed ofbarium borosilicate glass, alumino-borosilicate glass, or the like canbe used

In the case where a glass substrate is used and the temperature of heattreatment performed later is high, a glass substrate whose strain pointis higher than or equal to 730° C. is preferably used as the glasssubstrate. As a glass substrate, a glass material such asaluminosilicate glass, aluminoborosilicate glass, or barium borosilicateglass is used, for example. Note that in the case where a larger amountof barium oxide (BaO) than boron oxide is contained, a more practicalheat-resistant glass substrate can be obtained. Thus, a glass substratecontaining a larger amount of BaO than B₂O₃ is preferably used.

Note that a substrate formed of an insulator such as a ceramicsubstrate, a quartz substrate, or a sapphire substrate may be usedinstead of the above glass substrate. Crystallized glass or the like maybe used. A plastic substrate or the like can be used as appropriate.

First, the insulating layer 407 which serves as a base film is formedover the substrate 400 having an insulating surface. As the insulatinglayer 407 which is to be in contact with the oxide semiconductor layer,an oxide insulating layer such as a silicon oxide layer, a siliconoxynitride layer, an aluminum oxide layer, or an aluminum oxynitridelayer is preferably used. Although a plasma CVD method, a sputteringmethod, or the like can be used as a formation method of the insulatinglayer 407, a sputtering method is preferably used so that a large amountof hydrogen is prevented from being contained in the insulating layer407.

In this embodiment, as the insulating layer 407, a silicon oxide layeris formed over the substrate 400 by a sputtering method in such a mannerthat the substrate 400 is transferred to a treatment chamber, asputtering gas from which hydrogen and moisture are removed and whichcontains high-purity oxygen is introduced thereto, and a siliconsemiconductor target is used. The temperature of the substrate 400 maybe room temperature or the substrate 400 may be heated.

For example, the silicon oxide film is formed by an RF sputtering methodin an atmosphere containing oxygen and argon (flow rate of oxygen of 25sccm: flow rate of argon of 25 sccm=1:1) under the following conditions:the substrate temperature is 108° C., the distance between the substrateand the target (T-S distance) is 60 mm, the pressure is 0.4 Pa, and thehigh-frequency power is 1.5 kW. The thickness is 100 nm. Note that as atarget for forming the silicon oxide film, a silicon target can be usedinstead of quartz (preferably synthetic quartz). Note that oxygen or amixed gas of oxygen and argon is used as a sputtering gas.

In that case, the insulating layer 407 is preferably formed whilemoisture remaining in the treatment chamber is removed so that hydrogen,a hydroxyl group, or moisture is prevented from being contained in theinsulating layer 407.

In order to remove moisture remaining in the treatment chamber, anentrapment vacuum pump is preferably used. For example, a cryopump, anion pump, or a titanium sublimation pump is preferably used. Theevacuation unit may be a turbo pump provided with a cold trap. In thetreatment chamber which is evacuated with the cryopump, a hydrogen atom,a compound containing a hydrogen atom such as water (H₂O), a compoundcontaining a carbon atom, and the like are removed, whereby theconcentration of an impurity in the insulating layer 407 formed in thetreatment chamber can be reduced.

As a sputtering gas used for forming the insulating layer 407, ahigh-purity gas is preferably used in which impurities such as hydrogen,water, a hydroxyl group, or hydride are reduced to such a level that theimpurity concentration is represented by the unit “ppm” or “ppb”.

Examples of a sputtering method include an RF sputtering method in whicha high-frequency power source is used for a sputtering power supply, aDC sputtering method in which a DC power source is used, and a pulsed DCsputtering method in which a bias is applied in a pulsed manner. An RFsputtering method is mainly used in the case where an insulating film isformed, and a DC sputtering method is mainly used in the case where ametal film is formed.

In addition, there is also a multi-source sputtering apparatus in whicha plurality of targets of different materials can be set. With themulti-source sputtering apparatus, films of different materials can beformed to be stacked in the same chamber, or a film of plural kinds ofmaterials can be formed by electric discharge at the same time in thesame chamber.

In addition, there are a sputtering apparatus provided with a magnetsystem inside the chamber and used for a magnetron sputtering method, ora sputtering apparatus used for an ECR sputtering method in which plasmagenerated with the use of microwaves is used without using glowdischarge.

Furthermore, as a deposition method using a sputtering method, there arealso a reactive sputtering method in which a target substance and asputtering gas component are chemically reacted with each other duringdeposition to form a thin compound film thereof, and a bias sputteringmethod in which voltage is also applied to a substrate duringdeposition.

The insulating layer 407 may have a stacked-layer structure. Forexample, the insulating layer 407 may have a structure in which anitride insulating layer such as a silicon nitride layer, a siliconnitride oxide layer, an aluminum nitride layer, or an aluminum nitrideoxide layer and the above oxide insulating layer are stacked over thesubstrate 400.

For example, a silicon nitride layer is formed in such a manner that asputtering gas from which hydrogen and moisture are removed and whichcontains high-purity nitrogen is introduced between the silicon oxidelayer and the substrate and a silicon target is used. Also in that case,in a manner similar to that of the silicon oxide layer, the siliconnitride layer is preferably formed while moisture remaining in atreatment chamber is removed.

Also in the case of forming the silicon nitride layer, the substrate maybe heated at the time of film formation.

In the case where the silicon nitride layer and the silicon oxide layerare stacked as the insulating layer 407, the silicon nitride layer andthe silicon oxide layer can be formed in the same treatment chamberusing the same silicon target. First, the silicon nitride layer isformed in such a manner that a gas containing nitrogen is introduced anda silicon target provided in the treatment chamber is used. Then, thesilicon oxide layer is formed in such a manner that the gas is switchedto a gas containing oxygen and the silicon target used for the siliconnitride layer is used. The silicon nitride layer and the silicon oxidelayer can be formed successively without being exposed to air; thus,impurities such as hydrogen or moisture can be prevented from beingattached to the surface of the silicon nitride layer.

Then, an oxide semiconductor film is formed to a thickness of greaterthan or equal to 2 nm and less than or equal to 200 nm over theinsulating layer 407.

In order to prevent hydrogen, a hydroxyl group, and moisture from beingcontained in the oxide semiconductor film as much as possible, it ispreferable that as pretreatment of the film formation, the substrate 400over which the insulating layer 407 is formed is preheated in apreheating chamber of a sputtering apparatus so that impurities such ashydrogen and moisture attached to the substrate 400 can be eliminatedand removed. As an evacuation unit provided for the preheating chamber,a cryopump is preferably used. This preheating step can be omitted. Thispreheating step may be performed on the substrate 400 over which thegate insulating layer 402 has not been formed yet. This preheating stepmay be performed in a similar manner on the substrate 400 over whichcomponents up to and including the source or drain electrode layer 415 aand the source or drain electrode layer 415 b are formed.

Note that before the oxide semiconductor film is formed by a sputteringmethod, dust attached to the surface of the insulating layer 407 ispreferably removed by reverse sputtering in which plasma is generated byintroduction of an argon gas. The reverse sputtering is a method inwhich voltage is applied to a substrate side, not to a target side,using a high-frequency power source in an argon atmosphere and plasma isgenerated in the vicinity of the substrate so that a substrate surfaceis modified. Note that a nitrogen atmosphere, a helium atmosphere, anoxygen atmosphere, or the like may be used instead of an argonatmosphere.

The oxide semiconductor film is formed by a sputtering method. Thefollowing can be used as the oxide semiconductor film: anIn—Ga—Zn—O-based non-single-crystal film, an In—Sn—Zn—O-based oxidesemiconductor film, an In—Al—Zn—O-based oxide semiconductor film, aSn—Ga—Zn—O-based oxide semiconductor film, an Al—Ga—Zn—O-based oxidesemiconductor film, a Sn—Al—Zn—O-based oxide semiconductor film, anIn—Zn—O-based oxide semiconductor film, an In—Sn—O-based oxidesemiconductor film, an Sn—Zn—O-based oxide semiconductor film, anAl—Zn—O-based oxide semiconductor film, an In—O-based oxidesemiconductor film, a Sn—O-based oxide semiconductor film, or aZn—O-based oxide semiconductor film. In this embodiment, the oxidesemiconductor film is formed using an In—Ga—Zn—O-based oxidesemiconductor target for film formation by a sputtering method. Theoxide semiconductor film can be formed by a sputtering method in a raregas (typically, argon) atmosphere, an oxygen atmosphere, or anatmosphere including a rare gas (typically, argon) and oxygen. In thecase of using a sputtering method, the oxide semiconductor film may beformed using a target containing SiO₂ at greater than or equal to 2 wt %and less than or equal to 10 wt %.

As the sputtering gas used for forming the oxide semiconductor film, ahigh-purity gas is preferably used in which impurities such as hydrogen,water, a hydroxyl group, or hydride are reduced to such a level that theimpurity concentration is represented by the unit “ppm” or “ppb”.

As the target for forming the oxide semiconductor film by a sputteringmethod, a metal oxide target containing zinc oxide as a main componentcan be used. As another example of the metal oxide target, an oxidesemiconductor target for film formation containing In, Ga, and Zn (thecomposition ratio: In₂O₃:Ga₂O₃:ZnO=1:1:1 (mol ratio), In:Ga:Zn=1:1:0.5(atomic ratio)) can be used. As the oxide semiconductor target for filmformation containing In, Ga, and Zn, a target having a composition ratioof In:Ga:Zn=1:1:1 (atomic ratio) or In:Ga:Zn=1:1:2 (atomic ratio) can beused. In addition, the filling factor of the oxide semiconductor targetfor film formation is greater than or equal to 90% and less than orequal to 100%, preferably greater than or equal to 95% and less than orequal to 99.9%. An oxide semiconductor film which is formed using anoxide semiconductor target for film formation of, which has a highfilling factor, is dense.

The oxide semiconductor film is formed over the substrate 400 in such amanner that the substrate is held in a treatment chamber maintained atreduced pressure, a sputtering gas from which hydrogen and moisture areremoved is introduced while moisture remaining in the treatment chamberis removed, and metal oxide is used as a target. In order to removemoisture remaining in the treatment chamber, an entrapment vacuum pumpis preferably used. For example, a cryopump, an ion pump, or a titaniumsublimation pump is preferably used. The evacuation unit may be a turbopump provided with a cold trap. In the treatment chamber which isevacuated with the cryopump, a hydrogen atom, a compound containing ahydrogen atom such as water (H₂O) (preferably also a compound containinga carbon atom), and the like are removed, whereby the concentration ofan impurity in the oxide semiconductor film formed in the treatmentchamber can be reduced. The substrate may be heated at the time of theformation of the oxide semiconductor film.

As an example of the deposition condition, the substrate temperature isroom temperature, the distance between the substrate and the target is110 mm, the pressure is 0.4 Pa, the direct-current (DC) power is 0.5 kW,and the atmosphere is an atmosphere containing oxygen and argon (flowrate of oxygen of 15 sccm:flow rate of argon of 30 sccm). Note that apulsed direct-current (DC) power source is preferably used, in whichcase powder substances (also referred to as particles or dust) that areformed in deposition can be reduced and the film thickness can beuniform. The oxide semiconductor film preferably has a thickness ofgreater than or equal to 5 nm and less than or equal to 30 nm. Note thatthe appropriate thickness differs depending on an oxide semiconductormaterial, and the thickness may be set as appropriate depending on thematerial.

Next, the oxide semiconductor film is processed into an island-shapedoxide semiconductor layer 412 in a first photolithography step (see FIG.6A). Further, a resist mask for forming the island-shaped oxidesemiconductor layer 412 may be formed by an ink jet method. Formation ofthe resist mask by an inkjet method needs no photomask, which results ina reduction in manufacturing costs.

Note that the etching of the oxide semiconductor film here may beperformed by dry etching, wet etching, or both wet etching and dryetching.

As the etching gas for dry etching, a gas containing chlorine(chlorine-based gas such as chlorine (Cl₂), boron chloride (BCl₃),silicon chloride (SiCl₄), or carbon tetrachloride (CCl₄)) is preferablyused.

Alternatively, a gas containing fluorine (fluorine-based gas such ascarbon tetrafluoride (CF₄), sulfur hexafluoride (SF₆), nitrogentrifluoride (NF₃), or trifluoromethane (CHF₃)); hydrogen bromide (HBr);oxygen (O₂); any of these gases to which a rare gas such as helium (He)or argon (Ar) is added; or the like can be used.

As the dry etching method, a parallel plate RIE (reactive ion etching)method or an ICP (inductively coupled plasma) etching method can beused. In order to etch the films into desired shapes, the etchingcondition (the amount of electric power applied to a coil-shapedelectrode, the amount of electric power applied to an electrode on asubstrate side, the temperature of the electrode on the substrate side,or the like) is adjusted as appropriate.

As an etchant used for wet etching, a mixed solution of phosphoric acid,acetic acid, and nitric acid, an ammonia hydrogen peroxide mixture(hydrogen peroxide solution of 31 weight %:ammonia solution of 28 weight%:water=5:2:2), or the like can be used. Alternatively, ITO07N (producedby KANTO CHEMICAL CO., INC.) may be used.

The etchant after the wet etching is removed together with the etchedmaterials by cleaning. The waste liquid containing the etchant and thematerial etched off may be purified and the material may be reused. Whena material such as indium contained in the oxide semiconductor layer iscollected from the waste liquid after the etching and reused, theresources can be efficiently used and the cost can be reduced.

The etching conditions (such as an etchant, etching time, andtemperature) are adjusted as appropriate depending on the material sothat the oxide semiconductor film can be etched into a desired shape.

In this embodiment, the oxide semiconductor film is processed into theisland-shaped oxide semiconductor layer 412 by wet etching using a mixedsolution of phosphoric acid, acetic acid, and nitric acid as an etchant.

In this embodiment, first heat treatment is performed on the oxidesemiconductor layer 412. The temperature of the first heat treatment ishigher than or equal to 400° C. and lower than or equal to 750° C.,preferably higher than or equal to 400° C. and lower than the strainpoint of the substrate. Here, the substrate is put in an electricfurnace that is a kind of heat treatment apparatus and heat treatment isperformed on the oxide semiconductor layer in a nitrogen atmosphere at450° C. for one hour, and then water and hydrogen are prevented fromentering the oxide semiconductor layer with the oxide semiconductorlayer not exposed to air. In the above manner, the oxide semiconductorlayer is obtained. This first heat treatment makes it possible todehydrate or dehydrogenate the oxide semiconductor layer 412.

The heat treatment apparatus is not limited to an electric furnace andmay be provided with a device that heats an object to be processed bythermal conduction or thermal radiation from a heater such as aresistance heater or the like. For example, an RTA (rapid thermalanneal) apparatus such as a GRTA (gas rapid thermal anneal) apparatus oran LRTA (lamp rapid thermal anneal) apparatus can be used. An LRTAapparatus is an apparatus for heating an object to be processed byradiation of light (an electromagnetic wave) emitted from a lamp such asa halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arclamp, a high pressure sodium lamp, or a high pressure mercury lamp. AGRTA apparatus is an apparatus with which heat treatment is performedusing a high-temperature gas. As the gas, an inert gas which does notreact with a process object by heat treatment, such as nitrogen or arare gas such as argon is used.

For example, as the first heat treatment, GRTA may be performed asfollows. The substrate is transferred and put in an inert gas which hasbeen heated to a high temperature of 650° C. to 700° C., heated forseveral minutes, and transferred and taken out of the inert gas whichhas been heated to a high temperature. GRTA enables a high-temperatureheat treatment in a short time.

Note that in the first heat treatment, it is preferable that water,hydrogen, and the like be not contained in the atmosphere of nitrogen ora rare gas such as helium, neon, or argon. It is preferable that thepurity of nitrogen or the rare gas such as helium, neon, or argon whichis introduced into a heat treatment apparatus be set to be 6N (99.9999%)or higher, preferably 7N (99.99999%) or higher (that is, the impurityconcentration is 1 ppm or lower, preferably 0.1 ppm or lower).

Further, the oxide semiconductor layer may be crystallized to be amicrocrystalline film or a polycrystalline film depending on a conditionof the first heat treatment or a material of the oxide semiconductorlayer. For example, the oxide semiconductor layer may be crystallized tobe a microcrystalline semiconductor film having a degree ofcrystallization of 90% or more, or 80% or more. Further, depending onthe condition of the first heat treatment and the material of the oxidesemiconductor layer, the oxide semiconductor layer may be an amorphousoxide semiconductor film containing no crystalline component. The oxidesemiconductor layer may become an oxide semiconductor film in which amicrocrystalline portion (with a grain diameter greater than or equal to1 nm and greater than or less than 20 nm, typically greater than orequal to 2 nm and greater than or less than 4 nm) is mixed into anamorphous oxide semiconductor.

The first heat treatment for the oxide semiconductor layer can beperformed before the oxide semiconductor film is processed into theisland-shaped oxide semiconductor layers. In that case, the substrate istaken out from the heat apparatus after the first heat treatment, andthen a photolithography step is performed.

The heat treatment which has an effect of dehydrating or dehydrogenatingthe oxide semiconductor layer may be performed at any of the followingtimings: after the oxide semiconductor layer is formed; after the sourceelectrode and the drain electrode are formed over the oxidesemiconductor layer; and after the gate insulating film is formed overthe source electrode and the drain electrode.

Next, a conductive film is formed over the insulating layer 407 and theoxide semiconductor layer 412. The conductive film may be formed by asputtering method or a vacuum evaporation method. As examples of amaterial of the metal conductive film, an element selected from Al, Cr,Cu, Ta, Ti, Mo, or W; an alloy containing any of these elements as acomponent; an alloy film containing any of these elements incombination; and the like can be given. Alternatively, one or morematerials selected from manganese, magnesium, zirconium, beryllium, andthorium may be used. Further, the conductive film may have asingle-layer structure or a stacked-layer structure including two ormore layers. For example, a single-layer structure of an aluminum filmincluding silicon, a two-layer structure in which a titanium film isstacked over an aluminum film, a three-layer structure in which atitanium film, an aluminum film, and a titanium film are stacked in thisorder, and the like can be given. Alternatively, a film, an alloy film,or a nitride film which contains aluminum (Al) and one or a plurality ofelements selected from titanium (Ti), tantalum (Ta), tungsten (W),molybdenum (Mo), chromium (Cr), neodymium (Nd), and scandium (Sc) may beused.

In a second photolithography step, a resist mask is formed over theconductive film and the conductive film is selectively etched so thatthe source or drain electrode layer 415 a and the source or drainelectrode layer 415 b are formed, and then the resist mask is removed(see FIG. 6B). Note that edges of the formed source electrode layer andthe drain electrode layer are preferably in a tapered shape, in whichcase a gate insulating layer which is to be stacked over the source anddrain electrode layers can cover the source and drain electrode layersmore adequately.

In this embodiment, as the source or drain electrode layer 415 a and thesource or drain electrode layer 415 b, a titanium film with a thicknessof 150 nm is formed by a sputtering method.

Note that in order to prevent the oxide semiconductor layer 412 frombeing removed and the insulating layer 407 thereunder from being exposedin the etching of the conductive film, materials and etching conditionsof the conductive film and the oxide semiconductor layer 412 areadjusted as appropriate.

In this embodiment, a Ti film is used as the conductive film, anIn—Ga—Zn—O-based oxide semiconductor is used for the oxide semiconductorlayer 412, and an ammonia hydrogen peroxide solution (a mixture ofammonia, water, and a hydrogen peroxide solution) is used as an etchant.

Note that, in the second photolithography step, only part of the oxidesemiconductor layer 412 is etched to form an oxide semiconductor layerhaving a groove (a depressed portion) in some cases. In addition, theresist mask for forming the source or drain electrode layer 415 a andthe source or drain electrode layer 415 b may be formed by an ink-jetmethod. Formation of the resist mask by an inkjet method needs nophotomask, which results in a reduction in manufacturing costs.

Ultraviolet light, KrF laser light, or ArF laser light is used for lightexposure for forming the resist mask in the second photolithographystep. Channel length L of a thin film transistor to be completed lateris determined by a distance between bottom ends of the source electrodelayer and the drain electrode layer, which are adjacent to each otherover the oxide semiconductor layer 412. Note that in the case wherelight exposure is performed so that the channel length L becomes lessthan 25 nm, light exposure for forming the resist mask in the secondphotolithography step is performed using extreme ultraviolet with anextremely short wavelength of several nanometers to several tens ofnanometers. Light exposure using extreme ultraviolet enables highresolution and deep depth of focus. Thus, the channel length L of thethin film transistor to be completed later can be greater than or equalto 10 nm and less than or equal to 1000 nm and the operation speed of acircuit can be increased, and furthermore the value of off-state currentis extremely small, which leads to lower power consumption.

Next, the gate insulating layer 402 is formed over the insulating layer407, the oxide semiconductor layer 412, the source or drain electrodelayer 415 a, the source or drain electrode layer 415 b (see FIG. 6C).

Here, the oxide semiconductor that is made to be an intrinsic oxidesemiconductor or a substantially intrinsic oxide semiconductor (theoxide semiconductor that is highly purified) by removal of impurities isextremely sensitive to an interface state and an interface electriccharge; thus, an interface between the oxide semiconductor and the gateinsulating film is important. Therefore, the gate insulating film (GI)that is in contact with the highly-purified oxide semiconductor needs tohave higher quality.

For example, a high-density plasma CVD using a microwave (2.45 GHz) ispreferably used, in which case an insulating film which is dense, hashigh withstand voltage, and has high quality can be formed. Thehighly-purified oxide semiconductor and the high-quality gate insulatingfilm are in close contact with each other, whereby the interface statedensity can be reduced to obtain favorable interface characteristics.

In addition, since the insulating film formed using the high-densityplasma CVD apparatus can have a uniform thickness, the insulating filmhas excellent step coverage. In addition, as for the insulating filmformed using the high-density plasma CVD apparatus, the thickness of athin film can be controlled precisely.

Needless to say, another film formation method such as a sputteringmethod or a plasma CVD method can be employed as long as the methodenables formation of a good-quality insulating film as a gate insulatingfilm. Further, an insulating film whose film quality and characteristicof an interface between the insulating film and an oxide semiconductorare improved by heat treatment which is performed after formation of theinsulating film may be formed as a gate insulating film. In any case,any insulating film may be used as long as the insulating film hascharacteristics of enabling reduction in interface state density of aninterface between the insulating film and an oxide semiconductor andformation of a favorable interface as well as having favorable filmquality as a gate insulating film.

Further, when an oxide semiconductor containing impurities is subjectedto a gate bias-temperature stress test (BT test) for 12 hours underconditions that the temperature is 85° C. and the voltage applied to thegate is 2×10⁶ V/cm, a bond between the impurity and a main component ofthe oxide semiconductor is cleaved by a high electric field (B: bias)and a high temperature (T: temperature), and a generated dangling bondinduces drift of threshold voltage (V_(th)). In contrast, the presentinvention makes it possible to obtain a thin film transistor which isstable to a BT test by removal of impurities in an oxide semiconductor,especially hydrogen, water, and the like as much as possible to obtain afavorable characteristic of an interface between the oxide semiconductorfilm and a gate insulating film as described above.

The gate insulating layer can be formed to have a single-layer structureor a stacked-layer structure including a silicon oxide layer, a siliconnitride layer, a silicon oxynitride layer, a silicon nitride oxidelayer, and/or an aluminum oxide layer.

The gate insulating layer is formed with a high-density plasma CVDapparatus. Here, a high-density plasma CVD apparatus refers to anapparatus which can realize a plasma density higher than or equal to1×10¹¹/cm³. For example, plasma is generated by applying a microwavepower higher than or equal to 3 kW and lower than or equal to 6 kW sothat the insulating film is formed.

A monosilane gas (SiH₄), nitrous oxide (N₂O), and a rare gas areintroduced into a chamber as a source gas to generate high-densityplasma at a pressure higher than or equal to 10 Pa and lower than orequal to 30 Pa so that an insulating film is formed over a substratehaving an insulating surface, such as a glass substrate. After that, thesupply of a monosilane gas is stopped, and nitrous oxide (N₂O) and arare gas are introduced without exposure to the air, so that plasmatreatment may be performed on a surface of the insulating film. Theplasma treatment performed on the surface of the insulating film byintroducing nitrous oxide (N₂O) and a rare gas is performed at leastafter the insulating film is formed. The insulating film formed throughthe above process procedure has small thickness and corresponds to aninsulating film whose reliability can be ensured even though it has athickness less than 100 nm, for example.

The flow ratio of a monosilane gas (SiH₄) to nitrous oxide (N₂O) whichare introduced into the chamber is in the range of 1:10 to 1:200. Inaddition, as a rare gas which is introduced into the chamber, helium,argon, krypton, xenon, or the like can be used. In particular, argon,which is inexpensive, is preferably used.

The insulating film formed through the above process procedure isgreatly different from the insulating film formed using a conventionalparallel plate plasma CVD apparatus. The etching rate of the insulatingfilm formed through the above process procedure is lower than that ofthe insulating film formed using the conventional parallel plate plasmaCVD apparatus by 10% or more or 20% or more in the case where theetching rates with the same etchant are compared to each other. Thus, itcan be said that the insulating film formed using the high-densityplasma CVD apparatus is a dense film.

In this embodiment, a silicon oxynitride film (also referred to asSiO_(x)N_(y), where x>y>0) with a thickness of 100 nm is used as thegate insulating layer 402. The gate insulating layer 402 is formed insuch a manner that monosilane (SiH₄), nitrous oxide (N₂O), and argon(Ar) are used as film formation gases at flow rate ofSiH₄/N₂O/Ar=250/2500/2500 (sccm) in a high-density plasma CVD apparatus,and plasma is generated by applying a microwave power of 5 kW at a filmformation pressure of 30 Pa and a film formation temperature of 325° C.

Alternatively, the gate insulating layer 402 may be formed by asputtering method. In the case where a silicon oxide film is formed by asputtering method, a silicon target or a quartz target is used as atarget, and oxygen or a mixed gas of oxygen and argon is used as asputtering gas. The use of a sputtering method makes it possible toprevent a large amount of hydrogen from being contained in the gateinsulating layer 402.

The gate insulating layer 402 can have a structure in which a siliconoxide layer and a silicon nitride layer are stacked over the source ordrain electrode layer 415 a and the source or drain electrode layer 415b. For example, a silicon oxide layer (SiO_(x) (x>0)) with a thicknessof greater than or equal to 5 nm and less than or equal to 300 nm may beformed as a first gate insulating layer, and a silicon nitride layer(SiN_(y) (y>0) with a thickness of greater than or equal to 50 nm andless than or equal to 200 nm may be stacked as a second gate insulatinglayer over the first gate insulating layer by a sputtering method. Forexample, the silicon oxide layer with a thickness of 100 nm can beformed by an RF sputtering method in an atmosphere containing oxygen andargon (flow rate of oxygen of 25 sccm:flow rate of argon of 25 sccm=1:1)under conditions that the pressure is 0.4 Pa and the high-frequencypower is 1.5 kW.

Next, a resist mask is formed in a third photolithography step, the gateinsulating layer 402 is selectively etched so that parts of the gateinsulating layer 402 are removed, whereby openings 421 a and 421 breaching the source or drain electrode layer 415 a and the source ordrain electrode layer 415 b, respectively, are formed (see FIG. 6D).

Next, a conductive film is formed over the gate insulating layer 402 andin and over the openings 421 a and 421 b, and then the gate electrodelayer 411 and the wirings layers 414 a and 414 b are formed in a fourthphotolithography step. Note that a resist mask may be formed by aninkjet method. Formation of the resist mask by an inkjet method needs nophotomask, which results in a reduction in manufacturing costs.

The gate electrode layer 411 and the wiring layers 414 a and 414 b canbe formed to have a single-layer or stacked-layer structure including ametal material such as molybdenum, titanium, chromium, tantalum,tungsten, aluminum, copper, neodymium, or scandium, or an alloy materialwhich contains any of these materials as its main component.

For example, as a two-layer structure of the gate electrode layer 411,and the wiring layers 414 a and 414 b, the following structures arepreferable: a two-layer structure of an aluminum layer and a molybdenumlayer stacked thereover, a two-layer structure of a copper layer and amolybdenum layer stacked thereover, a two-layer structure of a copperlayer and a titanium nitride layer or a tantalum nitride layer stackedthereover, and a two-layer structure of a titanium nitride layer and amolybdenum layer. As a three-layer structure, a stack of a tungstenlayer or a tungsten nitride layer, a layer of an alloy of aluminum andsilicon or an alloy of aluminum and titanium, and a titanium nitridelayer or a titanium layer is preferable. Note that the gate electrodelayer can also be formed using a light-transmitting conductive film. Asan example of a material of the light-transmitting conductive film, alight-transmitting conductive oxide or the like can be given.

In this embodiment, a titanium film with a thickness of 150 nm is formedby a sputtering method as the gate electrode layer 411 and the wiringlayers 414 a and 414 b.

Next, second heat treatment (preferably at higher than or equal to 200°C. and lower than or equal to 400° C., for example higher than or equalto 250° C. and lower than or equal to 350° C.) is performed in an inertgas atmosphere or an oxygen gas atmosphere. In this embodiment, thesecond heat treatment is performed at 250° C. in a nitrogen atmospherefor one hour. Alternatively, the second heat treatment may be performedafter a protective insulating layer or a planarization insulating layeris formed over the thin film transistor 410.

Furthermore, heat treatment may be performed at 100° C. to 200° C. for 1hour to 30 hours in an air atmosphere. This heat treatment may beperformed at a fixed heating temperature. Alternatively, the followingchange in the heating temperature may be conducted plural timesrepeatedly: the heating temperature is increased from a room temperatureto a temperature of 100° C. to 200° C. and then decreased to a roomtemperature. Alternatively, this heat treatment may be performed beforeformation of the oxide insulating layer under reduced pressure. When theheat treatment is performed under reduced pressure, the heat treatmenttime can be shortened.

Through the above-described steps, the thin film transistor 410including the oxide semiconductor layer 412 in which the concentrationof hydrogen, moisture, hydride, and hydroxide are reduced can bemanufactured (see FIG. 6E). The thin film transistor 410 can be used asthe thin film transistor 106 in Embodiment 1.

Further, a protective insulating layer or a planarization insulatinglayer for planarization may be provided over the thin film transistor410. For example, the protective insulating layer can be formed to havea single-layer structure or a stacked-layer structure including asilicon oxide layer, a silicon nitride layer, a silicon oxynitridelayer, a silicon nitride oxide layer, and/or an aluminum oxide layer.

The planarization insulating layer can be formed using an organicmaterial having heat resistance, such as polyimide, acrylic,benzocyclobutene, polyamide, or epoxy. Other than such organicmaterials, it is also possible to use a low-dielectric constant material(a low-k material), a siloxane-based resin, PSG (phosphosilicate glass),BPSG (borophosphosilicate glass), or the like. The planarizationinsulating layer may be formed by stacking a plurality of insulatingfilms formed of these materials.

Note that the siloxane-based resin corresponds to a resin including aSi—O—Si bond formed using a siloxane-based material as a startingmaterial. The siloxane-based resin may include as a substituent anorganic group (e.g., an alkyl group or an aryl group) or a fluoro group.In addition, the organic group may include a fluoro group.

There is no particular limitation on the method for forming theplanarization insulating layer. The planarization insulating layer canbe formed, depending on the material, by a method such as a sputteringmethod, an SOG method, spin coating, dip coating, spray coating, or adroplet discharge method (such as an inkjet method, screen printing,offset printing, or the like), or a tool (equipment) such as a doctorknife, a roll coater, a curtain coater, or a knife coater.

As described above, in forming the oxide semiconductor film, moistureremaining in the reaction atmosphere is removed, whereby theconcentration of hydrogen and hydride in the oxide semiconductor filmcan be reduced. Thus, stabilization of the oxide semiconductor film canbe achieved.

As described above, the highly-purified oxide semiconductor layer isused in the thin film transistor, whereby the thin film transistor withreduced off-state current can be provided. In addition, the thin filmtransistor with reduced off-state current which is described in thisembodiment is applied to a pixel in a display device, whereby a periodin which a storage capacitor provided in the pixel can hold voltage canbe increased. Thus, a display device which consumes less power indisplaying a still image or the like can be provided.

This embodiment can be implemented in combination with any of thestructures of the other embodiments as appropriate.

(Embodiment 3)

In this embodiment, an example of a thin film transistor that can beapplied to the display device disclosed in this specification will bedescribed. Note that for portions that are the same as those inEmbodiment 2 and portions and steps that are similar to those inEmbodiment 2, Embodiment 2 can be referred to, and description thereofis not repeated. In addition, detailed description of the same parts isomitted. A thin film transistor 460 described in this embodiment can beused as the thin film transistor 106 in Embodiment 1.

An embodiment of a thin film transistor of this embodiment and anembodiment of a method for manufacturing the thin film transistor willbe described with reference to FIGS. 7A and 7B and FIGS. 8A to 8E.

FIG. 7A illustrates an example of a top surface structure of a thin filmtransistor and FIG. 7B illustrates an example of a cross-sectionalstructure thereof The thin film transistor 460 illustrated in FIGS. 7Aand 7B is a top-gate thin film transistor.

FIG. 7A is a top view of the top-gate thin film transistor 460 and FIG.7B is a cross-sectional view taken along line D1-D2 in FIG. 7A.

The thin film transistor 460 includes, over a substrate 450 having aninsulating surface, an insulating layer 457, a source or drain electrodelayer 465 a (465 a 1 and 465 a 2), an oxide semiconductor layer 462, asource or drain electrode layer 465 b, a wiring layer 468, a gateinsulating layer 452, and a gate electrode layer 461 (461 a and 461 b).The source or drain electrode layer 465 a (465 a 1 and 465 a 2) iselectrically connected to a wiring layer 464 through the wiring layer468. In addition, although not illustrated, the source or drainelectrode layer 465 b is also electrically connected to the wiring layerthrough an opening provided in the gate insulating layer 452.

A process of manufacturing the thin film transistor 460 over thesubstrate 450 will be described with reference to FIGS. 8A to 8E.

First, the insulating layer 457 which serves as a base film is formedover the substrate 450 having an insulating surface.

In this embodiment, as the insulating layer 457, a silicon oxide layeris formed by a sputtering method. The silicon oxide layer is formed overthe substrate 450 in such a manner that the substrate 450 is transferredto a treatment chamber, a sputtering gas from which hydrogen andmoisture are removed and which contains high-purity oxygen is introducedthereto, and a silicon target or quartz (preferably synthetic quartz) isused. Note that oxygen or a mixed gas of oxygen and argon is used as asputtering gas.

For example, the silicon oxide film is formed by an RF sputtering methodin an atmosphere containing oxygen and argon (flow rate of oxygen of 25sccm:flow rate of argon of 25 sccm=1:1) under the following conditions:quartz (preferably synthetic quartz) whose purity is 6N is used, thesubstrate temperature is 108° C., the distance between the substrate andthe target (T-S distance) is 60 mm, the pressure is 0.4 Pa, and thehigh-frequency power is 1.5 kW. The thickness is 100 nm. Note that as atarget for forming the silicon oxide film, a silicon target can be usedinstead of quartz (preferably synthetic quartz).

In that case, the insulating layer 457 is preferably formed whilemoisture remaining in the treatment chamber is removed so that hydrogen,a hydroxyl group, or moisture is prevented from being contained in theinsulating layer 457. In the treatment chamber which is evacuated withthe cryopump, a hydrogen atom, a compound containing a hydrogen atomsuch as water (H₂O), a compound containing a carbon atom, and the likeare removed, whereby the concentration of an impurity in the insulatinglayer 457 formed in the treatment chamber can be reduced.

As a sputtering gas used for forming the insulating layer 457, ahigh-purity gas is preferably used in which impurities such as hydrogen,water, a hydroxyl group, or hydride are reduced to such a level that theimpurity concentration is represented by the unit “ppm” or “ppb”.

The insulating layer 457 may have a stacked-layer structure. Forexample, the insulating layer 457 may have a structure in which anitride insulating layer such as a silicon nitride layer, a siliconnitride oxide layer, an aluminum nitride layer, or an aluminum nitrideoxide layer and the above oxide insulating layer are stacked over thesubstrate 400.

For example, a silicon nitride layer is formed in such a manner that asputtering gas from which hydrogen and moisture are removed and whichcontains high-purity nitrogen is introduced between the silicon oxidelayer and the substrate and a silicon target is used. Also in that case,in a manner similar to that of the silicon oxide layer, the siliconnitride layer is preferably formed while moisture remaining in thetreatment chamber is removed.

Next, a conductive film is formed over the insulating layer 457, and ina first photolithography step, a resist mask is formed over theconductive film and the conductive film is selectively etched so thatthe source and drain electrode layers 465 a 1 and 465 a 2 are formed,and then the resist mask is removed (see FIG. 8A). Although the sourceand drain electrode layers 465 a 1 and 465 a 2 are separated in thecross-sectional view, they are a continuous film. Note that edges of theformed source electrode layer and the drain electrode layer arepreferably in a tapered shape, in which case a gate insulating layerwhich is to be stacked over the source and drain electrode layers cancover the source and drain electrode layers more adequately.

As a material of the source and drain electrode layers 465 a 1 and 465 a2, an element selected from Al, Cr, Cu, Ta, Ti, Mo, or W; an alloycontaining any of these elements as a component; an alloy filmcontaining any of these elements in combination; and the like can begiven. Alternatively, one or more materials selected from manganese,magnesium, zirconium, beryllium, and thorium may be used. Further, theconductive film may have a single-layer structure or a stacked-layerstructure including two or more layers. For example, a single-layerstructure of an aluminum film including silicon, a two-layer structurein which a titanium film is stacked over an aluminum film, a three-layerstructure in which a titanium film, an aluminum film, and a titaniumfilm are stacked in this order, and the like can be given.Alternatively, a film, an alloy film, or a nitride film which containsaluminum (Al) and one or a plurality of elements selected from titanium(Ti), tantalum (Ta), tungsten (W), molybdenum (Mo), chromium (Cr),neodymium (Nd), and scandium (Sc) may be used.

In this embodiment, as the source and drain electrode layers 465 a 1 and465 a 2, a titanium film with a thickness of 150 nm is formed by asputtering method.

Next, the oxide semiconductor layer 462 with a thickness of greater thanor equal to 2 nm and less than or equal to 200 nm is formed.

In a second photolithography step, the formed oxide semiconductor filmis processed into the island-shaped oxide semiconductor layer 462 (seeFIG. 8B). In this embodiment, the oxide semiconductor film is formed bya sputtering method using an In—Ga—Zn—O-based oxide semiconductor targetfor film formation.

The oxide semiconductor film is formed over the substrate 450 in such amanner that the substrate is held in a treatment chamber maintained atreduced pressure, a sputtering gas from which hydrogen and moisture areremoved is introduced while moisture remaining in the treatment chamberis removed, and metal oxide is used as a target. In order to removemoisture remaining in the treatment chamber, an entrapment vacuum pumpis preferably used. For example, a cryopump, an ion pump, or a titaniumsublimation pump is preferably used. The evacuation unit may be a turbopump provided with a cold trap. In the treatment chamber which isevacuated with the cryopump, a hydrogen atom, a compound containing ahydrogen atom such as H₂O (preferably also a compound containing acarbon atom), and the like are removed, whereby the concentration of animpurity in the oxide semiconductor film formed in the treatment chambercan be reduced. The substrate may be heated at the time of the formationof the oxide semiconductor film.

As a sputtering gas used for forming the oxide semiconductor film, ahigh-purity gas is preferably used in which impurities such as hydrogen,water, a hydroxyl group, or hydride are reduced to such a level that theimpurity concentration is represented by the unit “ppm” or “ppb”.

As an example of the deposition condition, the substrate temperature isroom temperature, the distance between the substrate and the target is110 mm, the pressure is 0.4 Pa, the direct-current (DC) power is 0.5 kW,and the atmosphere is an atmosphere containing oxygen and argon (flowrate of oxygen of 15 sccm:flow rate of argon of 30 sccm). Note that apulsed direct-current (DC) power source is preferably used, in whichcase powder substances (also referred to as particles or dust) generatedin film formation can be reduced and the film thickness can be uniform.The oxide semiconductor film preferably has a thickness of greater thanor equal to 5 nm and less than or equal to 30 nm. Note that theappropriate thickness differs depending on an oxide semiconductormaterial, and the thickness may be set as appropriate depending on thematerial.

In this embodiment, the oxide semiconductor film is processed into theisland-shaped oxide semiconductor layer 462 by wet etching using a mixedsolution of phosphoric acid, acetic acid, and nitric acid as an etchant.

In this embodiment, first heat treatment is performed on the oxidesemiconductor layer 462. The temperature of the first heat treatment ishigher than or equal to 400° C. and lower than or equal to 750° C.,preferably higher than or equal to 400° C. and lower than the strainpoint of the substrate. Here, the substrate is put in an electricfurnace that is a kind of heat treatment apparatus and heat treatment isperformed on the oxide semiconductor layer in a nitrogen atmosphere at450° C. for one hour, and then water and hydrogen are prevented fromentering the oxide semiconductor layer with the oxide semiconductorlayer not exposed to air. In the above manner, the oxide semiconductorlayer is obtained. This first heat treatment makes it possible todehydrate or dehydrogenate the oxide semiconductor layer 462.

The heat treatment apparatus is not limited to an electric furnace andmay be provided with a device that heats an object to be processed bythermal conduction or thermal radiation from a heater such as aresistance heater or the like. For example, an RTA (rapid thermalanneal) apparatus such as a GRTA (gas rapid thermal anneal) apparatus oran LRTA (lamp rapid thermal anneal) apparatus can be used. For example,as the first heat treatment, GRTA may be performed as follows. Thesubstrate is transferred and put in an inert gas which has been heatedto a high temperature of 650° C. to 700° C., heated for several minutes,and transferred and taken out of the inert gas which has been heated toa high temperature. GRTA enables a high-temperature heat treatment in ashort time.

Note that in the first heat treatment, it is preferable that water,hydrogen, and the like be not contained in the atmosphere of nitrogen ora rare gas such as helium, neon, or argon. It is preferable that thepurity of nitrogen or the rare gas such as helium, neon, or argon whichis introduced into a heat treatment apparatus be set to be 6N (99.9999%)or higher, preferably 7N (99.99999%) or higher (that is, the impurityconcentration is 1 ppm or lower, preferably 0.1 ppm or lower).

Further, the oxide semiconductor layer may be crystallized to be amicrocrystalline film or a polycrystalline film depending on a conditionof the first heat treatment or a material of the oxide semiconductorlayer.

The first heat treatment for the oxide semiconductor layer can beperformed before the oxide semiconductor film is processed into theisland-shaped oxide semiconductor layers. In that case, the substrate istaken out from the heat apparatus after the first heat treatment, andthen a photolithography step is performed.

The heat treatment which has an effect of dehydrating or dehydrogenatingthe oxide semiconductor layer may be performed at any of the followingtimings: after the oxide semiconductor layer is formed; after the sourceelectrode and the drain electrode are further formed over the oxidesemiconductor layer; and after the gate insulating film is formed overthe source electrode and the drain electrode.

Next, a conductive film is formed over the insulating layer 457 and theoxide semiconductor layer 462, and in a third photolithography step, aresist mask is formed over the conductive film and the conductive filmis selectively etched so that the source or drain electrode layer 465 band the wiring layer 468 are formed, and then the resist mask is removed(see FIG. 8C). The source or drain electrode layer 465 b and the wiringlayer 468 may be formed using a material and in a step which are similarto those of the source and drain electrode layers 465 a 1 and 465 a 2.

In this embodiment, as the source or drain electrode layer 465 b and thewiring layer 468, a titanium film with a thickness of 150 nm is formedby a sputtering method. In this embodiment, since the example in whichthe same titanium films are used for the source and drain electrodelayers 465 a 1 and 465 a 2 and the source or drain electrode layer 465 bis described, etching selectivity of the source or drain electrode layer465 a 1 or 465 a 2 with respect to the source or drain electrode layer465 b is not obtained. For that reason, the wiring layer 468 is providedover a part of the source or drain electrode layer 465 a 2 which is notcovered with the oxide semiconductor layer 462 so that the source anddrain electrode layers 465 a 1 and 465 a 2 are not etched at the time ofetching of the source or drain electrode layer 465 b. In the case wheredifferent materials which have high selectivity in etching steps areused for the source or drain electrode layers 465 a 1 and 465 a 2 andthe source or drain electrode layer 465 b, the wiring layer 468 whichprotects the source or drain electrode layer 465 a 2 at the time ofetching is not necessarily provided.

Note that in order to prevent the oxide semiconductor layer 462 frombeing removed at the time of the etching of the conductive film,materials and etching conditions of the conductive film and the oxidesemiconductor layer 462 are adjusted as appropriate.

In this embodiment, a Ti film is used as the conductive film, anIn—Ga—Zn—O-based oxide semiconductor is used for the oxide semiconductorlayer 462, and an ammonia hydrogen peroxide solution (a mixture ofammonia, water, and a hydrogen peroxide solution) is used as an etchant.

In the third photolithography step, only part of the oxide semiconductorlayer 462 is etched, whereby an oxide semiconductor layer having agroove (a depressed portion) is formed in some cases. A resist mask usedfor forming the source or drain electrode layer 465 b and the wiringlayer 468 may be formed by an inkjet method. Formation of the resistmask by an inkjet method needs no photomask, which results in areduction in manufacturing costs.

Next, the gate insulating layer 452 is formed over the insulating layer457, the oxide semiconductor layer 462, the source and drain electrodelayers 465 a 1 and 465 a 2, and the source or drain electrode layer 465b.

The gate insulating layer 452 can be formed to have a single-layerstructure or a stacked-layer structure including a silicon oxide layer,a silicon nitride layer, a silicon oxynitride layer, a silicon nitrideoxide layer, and/or an aluminum oxide layer by a plasma CVD method, asputtering method, or the like. The gate insulating layer 452 ispreferably formed by a sputtering method so that a large amount ofhydrogen is prevented from being contained in the gate insulating layer452. In the case where a silicon oxide film is formed by a sputteringmethod, a silicon target or a quartz target is used as a target, andoxygen or a mixed gas of oxygen and argon is used as a sputtering gas.

The gate insulating layer 452 can have a structure in which a siliconoxide layer and a silicon nitride layer are stacked over the source anddrain electrode layers 465 a 1 and 465 a 2 and the source or drainelectrode layer 465 b. In this embodiment, the silicon oxide layer witha thickness of 100 nm is formed by an RF sputtering method in anatmosphere containing oxygen and argon (flow rate of oxygen of 25sccm:flow rate of argon of 25 sccm=1:1) under conditions that thepressure is 0.4 Pa and the high-frequency power is 1.5 kW.

Next, a resist mask is formed in a fourth photolithography step, and thegate insulating layer 452 is selectively etched so that part of the gateinsulating layer 452 is removed, whereby an opening 423 reaching thewiring layer 468 is formed (see FIG. 8D). Although not illustrated, anopening reaching the source or drain electrode layer 465 b may be formedat the time of the formation of the opening 423. In this embodiment, anexample is described in which the opening reaching the source or drainelectrode layer 465 b is formed after an interlayer insulating layer isfurther stacked and a wiring layer for electrical connection is formedin the opening.

Next, a conductive film is formed over the gate insulating layer 452 andin and over the opening 423. Then, in a fifth photolithography, the gateelectrode layer 461 (461 a and 461 b) and the wiring layer 464 areformed. Note that a resist mask may be formed by an inkjet method.Formation of the resist mask by an inkjet method needs no photomask,which results in a reduction in manufacturing costs.

The gate electrode layer 461 (461 a and 461 b) and the wiring layer 464can be formed to have a single-layer or stacked-layer structureincluding a metal material such as molybdenum, titanium, chromium,tantalum, tungsten, aluminum, copper, neodymium, or scandium, or analloy material which contains any of these materials as its maincomponent.

In this embodiment, as the gate electrode layer 461 (461 a and 461 b)and the wiring layer 464, a titanium film with a thickness of 150 nm isformed by a sputtering method.

Next, second heat treatment (preferably at higher than or equal to 200°C. and lower than or equal to 400° C., for example higher than or equalto 250° C. and lower than or equal to 350° C.) is performed in an inertgas atmosphere or an oxygen gas atmosphere. In this embodiment, thesecond heat treatment is performed at 250° C. in a nitrogen atmospherefor one hour. Alternatively, the second heat treatment may be performedafter a protective insulating layer or a planarization insulating layeris formed over the thin film transistor 410.

Furthermore, heat treatment may be performed at 100° C. to 200° C. for 1hour to 30 hours in an air atmosphere. This heat treatment may beperformed at a fixed heating temperature. Alternatively, the followingchange in the heating temperature may be conducted plural timesrepeatedly: the heating temperature is increased from a room temperatureto a temperature of 100° C. to 200° C. and then decreased to a roomtemperature. Alternatively, this heat treatment may be performed beforeformation of the oxide insulating layer under reduced pressure. When theheat treatment is performed under reduced pressure, the heat treatmenttime can be shortened.

Through the above-described steps, the thin film transistor 460including the oxide semiconductor layer 462 in which the concentrationof hydrogen, moisture, hydride, and hydroxide are reduced can bemanufactured (see FIG. 8E). The thin film transistor 460 can be used asa thin film transistor used in each pixel in the pixel portion 202 ofEmbodiment 1.

A protective insulating layer or a planarization insulating layer forplanarization may be provided over the thin film transistor 460. Notethat although not illustrated, an opening reaching the source or drainelectrode layer 465 b is formed in the gate insulating layer 452, theprotective insulating layer, or the planarization insulating layer, anda wiring layer which is to be electrically connected to the source ordrain electrode layer 465 b is formed in the opening.

As described above, in forming the oxide semiconductor film, moistureremaining in the reaction atmosphere is removed, whereby theconcentration of hydrogen and hydride in the oxide semiconductor filmcan be reduced. Thus, stabilization of the oxide semiconductor film canbe achieved.

As described above, the highly-purified oxide semiconductor layer isused in the thin film transistor, whereby the thin film transistor withreduced off-state current can be provided. In addition, the thin filmtransistor with reduced off-state current which is described in thisembodiment is applied to a pixel in a display device, whereby a periodin which a storage capacitor provided in the pixel can hold voltage canbe increased. Thus, a display device which consumes less power indisplaying a still image or the like can be provided.

This embodiment can be implemented in combination with any of thestructures of the other embodiments as appropriate.

(Embodiment 4)

In this embodiment, another example of a thin film transistor that canbe applied to the display device disclosed in this specification will bedescribed. Note that Embodiment 2 may be referred to for descriptions ofportions that are the same as those in Embodiment 2 and portions andsteps that are similar to those in Embodiment 2, and description thereofis not repeated. In addition, detailed description of the same parts isomitted. Thin film transistors 425 and 426 described in this embodimentcan be used as the thin film transistor 106 in Embodiment 1.

The thin film transistors of this embodiment will be described withreference to FIGS. 9A and 9B.

FIGS. 9A and 9B illustrate examples of cross-sectional structures of thethin film transistors. Each of the thin film transistors 425 and 426illustrated in FIGS. 9A and 9B is a thin film transistor with astructure in which an oxide semiconductor layer is interposed between aconductive layer and a gate electrode layer.

In FIGS. 9A and 9B, a silicon substrate is used as a substrate, and thethin film transistors 425 and 426 are each provided over an insulatinglayer 422 provided over a silicon substrate 420.

In FIG. 9A, a conductive layer 427 is provided between the insulatinglayer 422 provided over the silicon substrate 420 and an insulatinglayer 407 so as to overlap with at least the entire oxide semiconductorlayer 412.

Note that FIG. 9B illustrates an example in which a conductive layerbetween the insulating layer 422 and the insulating layer 407 isprocessed into a conductive layer 424 by etching and overlaps with partof the oxide semiconductor layer 412, which includes at least a channelregion.

The conductive layers 427 and 424 may be formed of a metal materialwhich can withstand the temperature of heat treatment performed later;as such a metal material, an element selected from titanium (Ti),tantalum (Ta), tungsten (W), molybdenum (Mo), chromium (Cr), neodymium(Nd), and scandium (Sc), an alloy containing any of the above elementsas a component, an alloy film containing a combination of any of theseelements, a nitride containing any of the above elements as itscomponent, or the like can be used. In addition, the conductive layers427 and 424 may each have a single-layer structure or a stacked-layerstructure. For example, a single-layer structure of a tungsten layer, astacked-layer structure including a tungsten nitride layer and atungsten layer, or the like can be used.

Further, the potential of the conductive layers 427 and 424 may be thesame as or different from that of a gate electrode layer 411 of the thinfilm transistors 425 and 426. The conductive layers 427 and 424 can alsofunction as a second gate electrode layer. In addition, the potential ofthe conductive layers 427 and 424 may be fixed potential such as GND or0 V.

The conductive layers 427 and 424 makes it possible to control theelectric characteristics of the thin film transistors 425 and 426,respectively.

As described above, the highly-purified oxide semiconductor layer isused in the thin film transistor, whereby the thin film transistor withreduced off-state current can be provided. In addition, the thin filmtransistor with reduced off-state current which is described in thisembodiment is applied to a pixel in a display device, whereby a periodin which a storage capacitor provided in the pixel can hold voltage canbe increased. Thus, a display device which consumes less power indisplaying a still image or the like can be provided.

This embodiment can be implemented in combination with any of thestructures of the other embodiments as appropriate.

(Embodiment 5)

In this embodiment, another example of a thin film transistor that canbe applied to the display device disclosed in this specification will bedescribed. A thin film transistor 390 described in this embodiment canbe used as the thin film transistor 106 in Embodiment 1.

FIGS. 10A to 10E illustrate an example of a cross-sectional structure ofthe thin film transistor of this embodiment. The thin film transistor390 illustrated in FIGS. 10A to 10E has a bottom-gate structure and iscalled an inverted staggered thin film transistor.

Although the thin film transistor 390 is described as a single-gate thinfilm transistor, a multi-gate thin film transistor including a pluralityof channel formation regions can be manufactured when needed.

A process of manufacturing the thin film transistor 390 over a substrate394 will be described below with reference to FIGS. 10A to 10E.

First, a conductive film is formed over the substrate 394 having aninsulating surface, and then in a first photolithography step, a gateelectrode layer 391 is formed. Edges of the formed gate electrode layerare preferably in a tapered shape, in which case a gate insulating layerwhich is to be stacked over the source and drain electrode layers cancover the source and drain electrode layers more adequately. Note that aresist mask may be formed by an inkjet method. Formation of the resistmask by an inkjet method needs no photomask, which results in areduction in manufacturing costs.

Although there is no particular limitation on a substrate which can beused as the substrate 394 having an insulating surface, the substrateneeds to have heat resistance high enough to withstand at least heattreatment to be performed later. A glass substrate made of bariumborosilicate glass, aluminoborosilicate glass, or the like can be used.

In the case where a glass substrate is used and the temperature of theheat treatment performed later is high, a glass substrate whose strainpoint is higher than or equal to 730° C. is preferably used. For a glasssubstrate, a glass material such as aluminosilicate glass,aluminoborosilicate glass, or barium borosilicate glass is used, forexample. Note that in the case where a larger amount of barium oxide(BaO) than boron oxide is contained, a more practical heat-resistantglass substrate can be obtained. Thus, a glass substrate containing alarger amount of BaO than B₂O₃ is preferably used.

Note that a substrate formed of an insulator such as a ceramicsubstrate, a quartz substrate, or a sapphire substrate may be usedinstead of the above glass substrate. Alternatively, crystallized glassor the like may be used. Alternatively, a plastic substrate or the likecan be used as appropriate.

An insulating film serving as a base film may be provided between thesubstrate 394 and the gate electrode layer 391. The base film has afunction of preventing diffusion of an impurity element from thesubstrate 394, and can be formed to have a single-layer structure or astacked-layer structure including one or more of a silicon nitride film,a silicon oxide film, a silicon nitride oxide film, and a siliconoxynitride film.

The gate electrode layer 391 can be formed to have a single-layer orstacked-layer structure including a metal material such as molybdenum,titanium, chromium, tantalum, tungsten, aluminum, copper, neodymium, orscandium, or an alloy material containing any of these materials as amain component.

For example, as a two-layer structure of the gate electrode layer 391,the following structure is preferable: a structure in which a molybdenumlayer is stacked over an aluminum layer, a structure in which amolybdenum layer is stacked over a copper layer, a structure in which atitanium nitride layer or a tantalum nitride layer is stacked over acopper layer, a structure in which a titanium nitride layer and amolybdenum layer are stacked, or a structure in which a tungsten nitridelayer and a tungsten layer are stacked. As a three-layer structure, astructure in which a tungsten layer or a tungsten nitride layer, a layerof an alloy of aluminum and silicon or an alloy of aluminum andtitanium, and a titanium nitride layer or a titanium layer are stackedis preferable. Note that the gate electrode layer can also be formedusing a light-transmitting conductive film. As an example of thelight-transmitting conductive film, a light-transmitting conductiveoxide or the like can be given.

Next, a gate insulating layer 397 is formed over the gate electrodelayer 391.

The gate insulating layer 397 can be formed to have a single-layerstructure or a stacked-layer structure including a silicon oxide layer,a silicon nitride layer, a silicon oxynitride layer, a silicon nitrideoxide layer, and/or an aluminum oxide layer by a plasma CVD method, asputtering method, or the like. The gate insulating layer 397 ispreferably formed by a sputtering method so that a large amount ofhydrogen is prevented from being contained in the insulating layer 397.In the case where a silicon oxide film is formed by a sputtering method,a silicon target or a quartz target is used as a target, and oxygen or amixed gas of oxygen and argon is used as a sputtering gas.

The gate insulating layer 397 can have a structure in which a siliconnitride layer and a silicon oxide layer are stacked over the gateelectrode layer 391. For example, a silicon nitride layer (SiN_(y)(y>0)) with a thickness of greater than or equal to 50 nm and less thanor equal to 200 nm is formed as a first gate insulating layer by asputtering method and a silicon oxide layer (SiO_(x) (x>0)) with athickness of greater than or equal to 5 nm and less than or equal to 300nm is stacked as a second gate insulating layer over the first gateinsulating layer to form a gate insulating layer with a thickness of 100nm.

In order to prevent hydrogen, a hydroxyl group, and moisture from beingcontained in the gate insulating layer 397 and an oxide semiconductorfilm 393 as much as possible, it is preferable that as pretreatment ofthe film formation, the substrate 394 over which the gate electrodelayer 391 is formed is preheated in a preheating chamber of a sputteringapparatus, or the substrate 394 over which components up to andincluding the gate insulating layer 397 are formed is preheated so thatimpurities such as hydrogen and moisture attached to the substrate 394can be eliminated and removed. The temperature of the preheating ishigher than or equal to 100° C. and lower than or equal to 400° C.,preferably higher than or equal to 150° C. and lower than or equal to300° C. As an evacuation unit provided for the preheating chamber, acryopump is preferably used. This preheating step can be omitted. Thispreheating step may be similarly performed on the substrate 394 overwhich components up to and including a source electrode layer 395 a anda drain electrode layer 395 b are formed before formation of an oxideinsulating layer 396.

Next, an oxide semiconductor film 393 is formed to a thickness ofgreater than or equal to 2 nm and less than or equal to 200 nm over thegate insulating layer 397 (see FIG. 10A).

Note that before the oxide semiconductor film 393 is formed by asputtering method, dust attached to the surface of the insulating layer397 is preferably removed by reverse sputtering in which plasma isgenerated by introduction of an argon gas. The reverse sputtering is amethod in which voltage is applied to a substrate side, not to a targetside, using an RF power source in an argon atmosphere and plasma isgenerated in the vicinity of the substrate so that a substrate surfaceis modified. Note that a nitrogen atmosphere, a helium atmosphere, anoxygen atmosphere, or the like may be used instead of an argonatmosphere.

The oxide semiconductor film 393 is formed by a sputtering method. Thefollowing can be used as the oxide semiconductor film 393: anIn—Ga—Zn—O-based non-single-crystal film, an In—Sn—Zn—O-based oxidesemiconductor film, an In—Al—Zn—O-based oxide semiconductor film, aSn—Ga—Zn—O-based oxide semiconductor film, an Al—Ga—Zn—O-based oxidesemiconductor film, a Sn—Al—Zn—O-based oxide semiconductor film, anIn—Sn—O-based oxide semiconductor film, an In—Zn—O-based oxidesemiconductor film, an Sn—Zn—O-based oxide semiconductor film, anAl—Zn—O-based oxide semiconductor film, an In—O-based oxidesemiconductor film, a Sn—O-based oxide semiconductor film, or aZn—O-based oxide semiconductor film. In this embodiment, the oxidesemiconductor film 393 is formed using an In—Ga—Zn—O-based oxidesemiconductor target for film formation by a sputtering method. Theoxide semiconductor film 393 can be formed by a sputtering method in arare gas (typically, argon) atmosphere, an oxygen atmosphere, or anatmosphere including a rare gas (typically, argon) and oxygen. In thecase of using a sputtering method, the oxide semiconductor film may beformed using a target containing SiO₂ at greater than or equal to 2 wt %and less than or equal to 10 wt %.

As the target for forming the oxide semiconductor film 393 by asputtering method, a metal oxide target containing zinc oxide as a maincomponent can be used. As another example of the metal oxide target, anoxide semiconductor target for film formation containing In, Ga, and Zn(the composition ratio: In₂O₃:Ga₂O₃:ZnO=1:1:1 (mol ratio),In:Ga:Zn=1:1:0.5 (atomic %)) can be used. As the oxide semiconductortarget for film formation containing In, Ga, and Zn, a target having acomposition ratio of In:Ga:Zn=1:1:1 (atomic ratio) or In:Ga:Zn=1:1:2(atomic ratio) can be used. In addition, the filling factor of the oxidesemiconductor target for film formation is greater than or equal to 90%and less than or equal to 100%, preferably greater than or equal to 95%and less than or equal to 99.9%. An oxide semiconductor film which isformed using an oxide semiconductor target for film formation, which hasa high filling factor, is dense.

The oxide semiconductor film 393 is formed over the substrate 394 insuch a manner that the substrate is held in a treatment chambermaintained at reduced pressure and is heated to room temperature orlower than 400° C., then a sputtering gas from which hydrogen andmoisture are removed is introduced while moisture remaining in thetreatment chamber is removed and metal oxide is used as a target. Inorder to remove moisture remaining in the treatment chamber, anentrapment vacuum pump is preferably used. For example, a cryopump, anion pump, or a titanium sublimation pump is preferably used. Theevacuation unit may be a turbo pump provided with a cold trap. In thetreatment chamber which is evacuated with the cryopump, a hydrogen atom,a compound containing a hydrogen atom such as H₂O (preferably also acompound containing a carbon atom), and the like are removed, wherebythe concentration of an impurity in the oxide semiconductor film formedin the treatment chamber can be reduced. Sputtering film formation isperformed while moisture remaining in the treatment chamber is removedusing a cryopump, whereby the substrate temperature in forming the oxidesemiconductor film 393 can be room temperature or a temperature of lowerthan 400° C.

As an example of the deposition condition, the distance between thesubstrate and the target is 100 mm, the pressure is 0.6 Pa, thedirect-current (DC) power is 0.5 kW, and the atmosphere is an oxygenatmosphere (the proportion of oxygen flow: 100%). Note that a pulseddirect-current (DC) power source is preferably used, in which casepowder substances (also referred to as particles or dust) generated infilm formation can be reduced and the film thickness can be uniform. Theoxide semiconductor film preferably has a thickness of greater than orequal to 5 nm and less than or equal to 30 nm. Note that the appropriatethickness differs depending on an oxide semiconductor material, and thethickness may be set as appropriate depending on the material.

Examples of a sputtering method include an RF sputtering method in whicha high-frequency power source is used for a sputtering power supply, aDC sputtering method in which a DC power source is used, and a pulsed DCsputtering method in which a bias is applied in a pulsed manner. An RFsputtering method is mainly used in the case where an insulating film isformed, and a DC sputtering method is mainly used in the case where ametal film is formed.

In addition, there is also a multi-source sputtering apparatus in whicha plurality of targets of different materials can be set. With themulti-source sputtering apparatus, films of different materials can beformed to be stacked in the same chamber, or a film of plural kinds ofmaterials can be formed by electric discharge at the same time in thesame chamber.

In addition, there are a sputtering apparatus provided with a magnetsystem inside the chamber and used for a magnetron sputtering method, ora sputtering apparatus used for an ECR sputtering method in which plasmagenerated with the use of microwaves is used without using glowdischarge.

Furthermore, as a deposition method using a sputtering method, there arealso a reactive sputtering method in which a target substance and asputtering gas component are chemically reacted with each other duringdeposition to form a thin compound film thereof, and a bias sputteringmethod in which voltage is also applied to a substrate duringdeposition.

Next, in a second photolithography step, the oxide semiconductor film isprocessed into an island-shaped oxide semiconductor layer 399 (see FIG.10B). Note that a resist mask for forming the island-shaped oxidesemiconductor layer 399 may be formed by an inkjet method. Formation ofthe resist mask by an inkjet method needs no photomask, which results ina reduction in manufacturing costs.

In the case where a contact hole is formed in the gate insulating layer397, the contact hole can be formed at the time of the formation of theoxide semiconductor layer 399.

Note that the etching of the oxide semiconductor film 393 here may beperformed by dry etching, wet etching, or both wet etching and dryetching.

As the etching gas for dry etching, a gas containing chlorine(chlorine-based gas such as chlorine (Cl₂), boron chloride (BCl₃),silicon chloride (SiCl₄), or carbon tetrachloride (CCl₄)) is preferablyused.

Alternatively, a gas containing fluorine (fluorine-based gas such ascarbon tetrafluoride (CF₄), sulfur hexafluoride (SF₆), nitrogentrifluoride (NF₃), or trifluoromethane (CHF₃)); hydrogen bromide (HBr);oxygen (O₂); any of these gases to which a rare gas such as helium (He)or argon (Ar) is added; or the like can be used.

As the dry etching method, a parallel plate RIE (reactive ion etching)method or an ICP (inductively coupled plasma) etching method can beused. In order to etch the films into desired shapes, the etchingcondition (the amount of electric power applied to a coil-shapedelectrode, the amount of electric power applied to an electrode on asubstrate side, the temperature of the electrode on the substrate side,or the like) is adjusted as appropriate.

As an etchant used for wet etching, a mixed solution of phosphoric acid,acetic acid, and nitric acid, an ammonia hydrogen peroxide mixture(hydrogen peroxide solution of 31 weight %: ammonia solution of 28weight %:water=5:2:2), or the like can be used. Alternatively, ITO07N(produced by KANTO CHEMICAL CO., INC.) may be used.

The etchant after the wet etching is removed together with the etchedmaterials by cleaning. The waste liquid containing the etchant and thematerial etched off may be purified and the material may be reused. Whena material such as indium contained in the oxide semiconductor layer iscollected from the waste liquid after the etching and reused, theresources can be efficiently used and the cost can be reduced.

The etching conditions (such as an etchant, etching time, andtemperature) are adjusted as appropriate depending on the material sothat the oxide semiconductor film can be etched into a desired shape.

Note that before formation of a conductive film in the subsequent step,reverse sputtering is preferably performed so that a resist residue orthe like attached to surfaces of the oxide semiconductor layer 399 andthe gate insulating layer 397 is removed.

Next, a conductive film is formed over the gate insulating layer 397 andthe oxide semiconductor layer 399. The conductive film may be formed bya sputtering method or a vacuum evaporation method. As examples of amaterial of the metal conductive film, an element selected from Al, Cr,Cu, Ta, Ti, Mo, or W; an alloy containing any of these elements as acomponent; an alloy film containing any of these elements incombination; and the like can be given. Alternatively, one or morematerials selected from manganese, magnesium, zirconium, beryllium, andthorium may be used. Further, the conductive film may have asingle-layer structure or a stacked-layer structure including two ormore layers. For example, a single-layer structure of an aluminum filmincluding silicon, a two-layer structure in which a titanium film isstacked over an aluminum film, a three-layer structure in which atitanium film, an aluminum film, and a titanium film are stacked in thisorder, and the like can be given. Alternatively, a film, an alloy film,or a nitride film which contains aluminum (Al) and one or a plurality ofelements selected from titanium (Ti), tantalum (Ta), tungsten (W),molybdenum (Mo), chromium (Cr), neodymium (Nd), and scandium (Sc) may beused.

In a third photolithography step, a resist mask is formed over theconductive film and the conductive film is selectively etched so thatthe source electrode layer 395 a and the drain electrode layer 395 b areformed, and then the resist mask is removed (see FIG. 10C).

Ultraviolet light, KrF laser light, or ArF laser light is used for lightexposure for forming the resist mask in the third photolithography step.Channel length L of a thin film transistor to be completed later isdetermined by a distance between bottom ends of the source electrodelayer and the drain electrode layer, which are adjacent to each otherover the oxide semiconductor layer 399. Note that in the case wherelight exposure is performed so that the channel length L becomes lessthan 25 nm, light exposure for forming the resist mask in the thirdphotolithography step is performed using extreme ultraviolet with anextremely short wavelength of several nanometers to several tens ofnanometers. Light exposure using extreme ultraviolet enables highresolution and deep depth of focus. Thus, the channel length L of thethin film transistor to be completed later can be greater than or equalto 10 nm and less than or equal to 1000 nm and the operation speed of acircuit can be increased, and furthermore the value of off-state currentis extremely small, which leads to lower power consumption can beachieved.

Note that in order to prevent the oxide semiconductor layer 399 frombeing removed at the time of the etching of the conductive film,materials and etching conditions of the conductive film and the oxidesemiconductor layer 399 are adjusted as appropriate.

In this embodiment, a Ti film is used as the conductive film, anIn—Ga—Zn—O-based oxide semiconductor is used for the oxide semiconductorlayer 399, and an ammonia hydrogen peroxide solution (a mixture ofammonia, water, and a hydrogen peroxide solution) is used as an etchant.

Note that, in the third photolithography step, only part of the oxidesemiconductor layer 399 is etched to form an oxide semiconductor layerhaving a groove (a depressed portion) in some cases. In addition, theresist mask for forming the source electrode layer 395 a and the s drainelectrode layer 395 b may be formed by an ink-jet method. Formation ofthe resist mask by an inkjet method needs no photomask, which results ina reduction in manufacturing costs.

In order to reduce the number of photomasks and the number of steps in aphotolithography process, etching may be performed with the use of aresist mask formed using a multi-tone mask which is a light-exposuremask through which light is transmitted so as to have a plurality ofintensities. Since a resist mask formed using a multi-tone mask has aplurality of thicknesses and can be further changed in shape byperforming etching, the resist mask can be used in a plurality ofetching steps to provide different patterns. Therefore, a resist maskcorresponding to at least two kinds of different patterns can be formedby using a multi-tone mask. Thus, the number of light-exposure masks canbe reduced and the number of corresponding photolithography steps can bealso reduced, whereby simplification of a process can be realized.

Water or the like attached to a surface of the exposed oxidesemiconductor layer may be removed by plasma treatment using a gas suchas N₂O, N₂, or Ar. The plasma treatment may be performed using a mixedgas of oxygen and argon.

In the case where the plasma treatment is performed, an oxide insulatinglayer 396 is formed as an oxide insulating layer serving as a protectiveinsulating film in contact with part of the oxide semiconductor layerwithout being exposed to air (see FIG. 10C). In this embodiment, theoxide insulating layer 396 is formed so as to be in contact with theoxide semiconductor layer 399 in a region where the oxide semiconductorlayer 399 does not overlap with the source electrode layer 395 a and thedrain electrode layer 395 b.

In this embodiment, a silicon oxide layer containing a defect is formedas the oxide insulating layer 396 in such a manner that the substrate394 over which components up to and including the island-shaped oxidesemiconductor layer 399, the source electrode layer 395 a, and the drainelectrode layer 395 b are formed is heated to room temperature or lowerthan 100° C., a sputtering gas containing high-purity oxygen from whichhydrogen and moisture are removed is introduced, and a siliconsemiconductor target is used.

For example, the silicon oxide film is formed by a pulsed DC sputteringmethod using a boron-doped silicon target which has a purity of 6N (theresistivity is 0.01 Ωcm) in an atmosphere of oxygen (the proportion ofoxygen flow rate is 100%) under the following conditions: the distancebetween the substrate and the target (T-S distance) is 89 mm, thepressure is 0.4 Pa, and the direct-current (DC) power is 6 kW. The filmthickness is 300 nm. Note that as the target for forming the siliconoxide film, quartz (preferably synthetic quartz) can be used instead ofthe silicon target. Note that oxygen or a mixed gas of oxygen and argonis used as a sputtering gas.

In that case, the oxide insulating layer 396 is preferably formed whilemoisture remaining in the treatment chamber is removed so that hydrogen,a hydroxyl group, or moisture is prevented from being contained in theoxide semiconductor layer 399 and the oxide insulating layer 396.

In order to remove moisture remaining in the treatment chamber, anentrapment vacuum pump is preferably used. For example, a cryopump, anion pump, or a titanium sublimation pump is preferably used. Theevacuation unit may be a turbo pump provided with a cold trap. In thetreatment chamber which is evacuated with the cryopump, a hydrogen atom,a compound containing a hydrogen atom such as water (H₂O), a compoundcontaining a carbon atom, and the like are removed, whereby theconcentration of an impurity in the oxide insulating layer 396 formed inthe treatment chamber can be reduced.

Note that as the oxide insulating layer 396, a silicon oxynitride layer,an aluminum oxide layer, an aluminum oxynitride layer, or the like canbe used instead of the silicon oxide layer.

Furthermore, heat treatment may be performed at 100° C. to 400° C. withthe oxide insulating layer 396 and the oxide semiconductor layer 399being in contact with each other. The oxide insulating layer 396 in thisembodiment contains a large number of defects; thus, impurities such ashydrogen, moisture, a hydroxyl group, or hydride contained in the oxidesemiconductor layer 399 can be diffused into the oxide insulating layer396 by this heat treatment so that the impurities contained in the oxidesemiconductor layer 399 are reduced.

Through the above-described steps, the thin film transistor 390including the oxide semiconductor layer 392 in which the concentrationof hydrogen, moisture, a hydroxyl group, or hydride is reduced can bemanufactured (see FIG. 10E).

As described above, in forming the oxide semiconductor film, moistureremaining in the reaction atmosphere is removed, whereby theconcentration of hydrogen and hydride in the oxide semiconductor filmcan be reduced. Thus, stabilization of the oxide semiconductor film canbe achieved.

A protective insulating layer may be provided over the oxide insulatinglayer. In this embodiment, a protective insulating layer 398 is formedover the oxide insulating layer 396. A silicon nitride film, a siliconnitride oxide film, an aluminum nitride film, an aluminum nitride oxidefilm, or the like is used as the protective insulating layer 398.

As the protective insulating layer 398, a silicon nitride film is formedusing a silicon semiconductor target in such a manner that the substrate394 over which components up to and including the oxide insulating layer396 are formed is heated to a temperature of 100° C. to 400° C. and asputtering gas from which hydrogen and moisture are removed and whichcontains high-purity nitrogen is introduced. Also in that case, in amanner similar to that of the oxide insulating layer 396, the protectiveinsulating layer 398 is preferably formed while moisture remaining inthe treatment chamber is removed.

In the case where the protective insulating layer 398 is formed, thesubstrate 394 is heated to 100° C. to 400° C. at the time of theformation of the protective insulating layer 398, whereby hydrogen ormoisture contained in the oxide semiconductor layer can be diffused intothe oxide insulating layer. In that case, heat treatment does not haveto be performed after the formation of the oxide insulating layer 396.

In the case where the silicon oxide layer is formed as the oxideinsulating layer 396 and the silicon nitride layer as the protectiveinsulating layer 398 is stacked thereover, the silicon oxide layer andthe silicon nitride layer can be formed in the same treatment chamberusing the same silicon target. First, the silicon oxide layer is formedin such a manner that a gas containing oxygen is introduced and asilicon target provided in the treatment chamber is used. Then, thesilicon nitride layer is formed in such a manner that the gas isswitched to a gas containing nitrogen and the silicon target used forthe silicon nitride layer is used. The silicon oxide layer and thesilicon nitride layer can be formed successively without being exposedto air; thus, impurities such as hydrogen or moisture can be preventedfrom being attached to the surface of the silicon oxide layer. In thatcase, heat treatment (at a temperature of 100° C. to 400° C.) ispreferably performed so that hydrogen or moisture contained in the oxidesemiconductor layer is diffused into the oxide insulating layer afterthe silicon oxide layer is formed as the oxide insulating layer 396 andthe silicon nitride layer as the protective insulating layer 398 isstacked thereover.

Furthermore, heat treatment may be performed at 100° C. to 200° C. for 1hour to 30 hours in an air atmosphere after the formation of theprotective insulating layer. This heat treatment may be performed at afixed heating temperature. Alternatively, the following change in theheating temperature may be conducted plural times repeatedly: theheating temperature is increased from a room temperature to atemperature of 100° C. to 200° C. and then decreased to a roomtemperature. Alternatively, this heat treatment may be performed beforeformation of the oxide insulating layer under reduced pressure. When theheat treatment is performed under reduced pressure, the heat treatmenttime can be shortened. This heat treatment enables a normally-off thinfilm transistor to be obtained. Thus, the reliability of a displaydevice can be increased.

Further, moisture remaining in the reaction atmosphere is removed at thetime of forming the oxide semiconductor layer serving as a channelformation region over the gate insulating layer, whereby theconcentration of hydrogen and hydride in the oxide semiconductor layercan be reduced.

Since the above-described steps are performed at a temperature of 400°C. or lower, the process can be applied to a manufacturing process usinga glass substrate having a side longer than or equal to 1 meter and athickness less than or equal to 1 millimeter. In addition, since all ofthe above steps can be performed at a treatment temperature of 400° C.or lower, a display panel can be manufactured without consuming too muchenergy.

As described above, the highly-purified oxide semiconductor layer isused in the thin film transistor, whereby the thin film transistor withreduced off-state current can be provided. In addition, the thin filmtransistor with reduced off-state current which is described in thisembodiment is applied to a pixel in a display device, whereby a periodin which a storage capacitor provided in the pixel can hold voltage canbe increased. Thus, a display device which consumes less power indisplaying a still image or the like can be provided.

This embodiment can be implemented in combination with any of thestructures of the other embodiments as appropriate.

(Embodiment 6)

In this embodiment, another example of a thin film transistor that canbe applied to the display device disclosed in this specification will bedescribed. A thin film transistor 310 described in this embodiment canbe used as the thin film transistor 106 in Embodiment 1.

FIGS. 11A to 11E illustrate an example of a cross-sectional structure ofthe thin film transistor of this embodiment. The thin film transistor310 illustrated in FIGS. 11A to 11E has a bottom-gate structure and isalso called an inverted staggered thin film transistor.

Although the thin film transistor 310 is described as a single-gate thinfilm transistor, a multi-gate thin film transistor including a pluralityof channel formation regions can be manufactured when needed.

A process of manufacturing the thin film transistor 310 over a substrate300 will be described below with reference to FIGS. 11A to 11E. FIG. 11Eillustrates a structure in which a protective insulating layer is formedover the thin film transistor 310.

First, a conductive film is formed over the substrate 300 having aninsulating surface, and then a first photolithography step is performedthereon to form a gate electrode layer 311. Note that a resist mask maybe formed by an inkjet method. Formation of the resist mask by an inkjetmethod needs no photomask, which results in a reduction in manufacturingcosts.

Although there is no particular limitation on a substrate which can beused as the substrate 300 having an insulating surface, the substrateneeds to have heat resistance high enough to withstand at least heattreatment to be performed later. A glass substrate made of bariumborosilicate glass, aluminoborosilicate glass, or the like can be used.

In the case where a glass substrate is used and the temperature of theheat treatment performed later is high, a glass substrate whose strainpoint is higher than or equal to 730° C. is preferably used. For a glasssubstrate, a glass material such as aluminosilicate glass,aluminoborosilicate glass, or barium borosilicate glass is used, forexample. Note that in the case where a larger amount of barium oxide(BaO) than boron oxide is contained, a more practical heat-resistantglass substrate can be obtained. Thus, a glass substrate containing alarger amount of BaO than B₂O₃ is preferably used.

Note that a substrate formed of an insulator such as a ceramicsubstrate, a quartz substrate, or a sapphire substrate may be usedinstead of the above glass substrate. Alternatively, crystallized glassor the like may be used.

An insulating film serving as a base film may be provided between thesubstrate 300 and the gate electrode layer 311. The base film has afunction of preventing diffusion of an impurity element from thesubstrate 300, and can be formed to have a single-layer structure or astacked-layer structure including one or more of a silicon nitride film,a silicon oxide film, a silicon nitride oxide film, and a siliconoxynitride film.

The gate electrode layer 311 can be formed to have a single-layer orstacked-layer structure including a metal material such as molybdenum,titanium, chromium, tantalum, tungsten, aluminum, copper, neodymium, orscandium, or an alloy material containing any of these materials as amain component.

For example, as a two-layer structure of the gate electrode layer 311,the following structure is preferable: a structure in which a molybdenumlayer is stacked over an aluminum layer, a structure in which amolybdenum layer is stacked over a copper layer, a structure in which atitanium nitride layer or a tantalum nitride layer is stacked over acopper layer, a structure in which a titanium nitride layer and amolybdenum layer are stacked, or a structure in which a tungsten nitridelayer and a tungsten layer are stacked. As a three-layer structure, astructure in which a tungsten layer or a tungsten nitride layer, a layerof an alloy of aluminum and silicon or an alloy of aluminum andtitanium, and a titanium nitride layer or a titanium layer are stackedis preferable.

Next, a gate insulating layer 302 is formed over the gate electrodelayer 311.

The gate insulating layer 302 can be formed to have a single-layerstructure or a stacked-layer structure including a silicon oxide layer,a silicon nitride layer, a silicon oxynitride layer, a silicon nitrideoxide layer, and/or an aluminum oxide layer by a plasma CVD method, asputtering method, or the like. For example, a silicon oxynitride layercan be formed using a deposition gas containing SiH₄, oxygen, andnitrogen by plasma CVD. The thickness of the gate insulating layer 302is greater than or equal to 100 nm and less than or equal to 500 nm. Inthe case of a stacked-layer structure, for example, a first gateinsulating layer with a thickness of greater than or equal to 50 nm andless than or equal to 200 nm and a second gate insulating layer with athickness of greater than or equal to 5 nm and less than or equal to 300nm are stacked in this order.

In this embodiment, as the gate insulating layer 302, a siliconoxynitride layer with a thickness of less than or equal to 200 nm isformed by a plasma CVD method.

Next, an oxide semiconductor film 330 is formed to a thickness greaterthan or equal to 2 nm and less than or equal to 200 nm over the gateinsulating layer 302.

Note that before the oxide semiconductor film 330 is formed by asputtering method, dust on a surface of the second gate insulating layer302 is preferably removed by reverse sputtering in which plasma isgenerated by introduction of an argon gas. Note that a nitrogenatmosphere, a helium atmosphere, an oxygen atmosphere, or the like maybe used instead of an argon atmosphere.

The oxide semiconductor film 330 is formed using an In—Ga—Zn—O-basednon-single-crystal film; or an In—Sn—Zn—O-based oxide semiconductorfilm, an In—Al—Zn—O-based oxide semiconductor film, a Sn—Ga—Zn—O-basedoxide semiconductor film, an Al—Ga—Zn—O-based oxide semiconductor film,a Sn—Al—Zn—O-based oxide semiconductor film, an In—Sn—O-based oxidesemiconductor film, an In—Zn—O-based oxide semiconductor film, anSn—Zn—O-based oxide semiconductor film, an Al—Zn—O-based oxidesemiconductor film, an In—O-based oxide semiconductor film, a Sn—O-basedoxide semiconductor film, or a Zn—O-based oxide semiconductor film. Inthis embodiment, the oxide semiconductor film 330 is formed by asputtering method using an In—Ga—Zn—O-based oxide semiconductor targetfor film formation. A cross-sectional view at this stage corresponds toFIG. 11A. In addition, the oxide semiconductor film 330 can be formed bya sputtering method in a rare gas (typically, argon) atmosphere, anoxygen atmosphere, or an atmosphere containing a rare gas (typically,argon) and oxygen. In the case where the oxide semiconductor film 330 isformed by a sputtering method, it may be formed using a targetcontaining SiO₂ at 2 wt % to 10 wt %.

As the target for forming the oxide semiconductor film 330 by asputtering method, a metal oxide target containing zinc oxide as a maincomponent can be used. As another example of the metal oxide target, anoxide semiconductor target for film formation containing In, Ga, and Zn(the composition ratio: In₂O₃:Ga₂O₃:ZnO=1:1:1 (mol ratio),In:Ga:Zn=1:1:0.5 (atomic %)) can be used. As the oxide semiconductortarget for film formation containing In, Ga, and Zn, a target having acomposition ratio of In:Ga:Zn=1:1:1 (atomic ratio) or In:Ga:Zn=1:1:2(atomic ratio) can be used. In addition, the filling factor of the oxidesemiconductor target for film formation is greater than or equal to 90%and less than or equal to 100%, preferably greater than or equal to 95%and less than or equal to 99.9%. An oxide semiconductor film which isformed using an oxide semiconductor target for film formation, which hasa high filling factor, is dense.

As a sputtering gas used for forming the oxide semiconductor film 330, ahigh-purity gas is preferably used in which impurities such as hydrogen,water, a hydroxyl group, or hydride are reduced to such a level that theimpurity concentration is represented by the unit “ppm” or “ppb”.

The substrate is held in a treatment chamber maintained at reducedpressure and is heated to higher than or equal to 100° C. and lower thanor equal to 600° C., preferably higher than or equal to 200° C. andlower than or equal to 400° C. Film formation is performed while thesubstrate is heated, whereby the concentration of impurities containedin the formed oxide semiconductor film can be reduced. Then, asputtering gas from which hydrogen and moisture are removed isintroduced while moisture remaining in the treatment chamber is removed,and the oxide semiconductor film 330 is formed over the gate insulatinglayer 302 using metal oxide as a target. In order to remove moistureremaining in the treatment chamber, an entrapment vacuum pump ispreferably used. For example, a cryopump, an ion pump, or a titaniumsublimation pump is preferably used. The evacuation unit may be a turbopump provided with a cold trap. In the treatment chamber which isevacuated with the cryopump, a hydrogen atom, a compound containing ahydrogen atom such as H₂O (preferably also a compound containing acarbon atom), and the like are removed, whereby the concentration of animpurity in the oxide semiconductor film formed in the treatment chambercan be reduced.

As an example of the deposition condition, the distance between thesubstrate and the target is 100 mm, the pressure is 0.6 Pa, thedirect-current (DC) power is 0.5 kW, and the atmosphere is an oxygenatmosphere (the proportion of oxygen flow: 100%). Note that a pulseddirect-current (DC) power source is preferably used, in which casepowder substances (also referred to as particles or dust) generated infilm formation can be reduced and the film thickness can be uniform. Theoxide semiconductor film preferably has a thickness of greater than orequal to 5 nm and less than or equal to 30 nm. Note that the appropriatethickness differs depending on an oxide semiconductor material, and thethickness may be set as appropriate depending on the material.

Next, the oxide semiconductor film 330 is processed into anisland-shaped oxide semiconductor layer in a second photolithographystep. A resist mask for forming the island-shaped oxide semiconductorlayer may be formed by an inkjet method. Formation of the resist mask byan inkjet method needs no photomask, which results in a reduction inmanufacturing costs.

Next, first heat treatment is performed on the oxide semiconductorlayer. The oxide semiconductor layer can be dehydrated or dehydrogenatedby this first heat treatment. The temperature of the first heattreatment is higher than or equal to 400° C. and lower than or equal to750° C., preferably higher than or equal to 400° C. and lower than thestrain point of the substrate. Here, the substrate is put in an electricfurnace that is a kind of heat treatment apparatus and heat treatment isperformed on the oxide semiconductor layer in a nitrogen atmosphere at450° C. for one hour, and then water and hydrogen are prevented fromentering the oxide semiconductor layer with the oxide semiconductorlayer not exposed to air. In the above manner, the oxide semiconductorlayer 331 is obtained (see FIG. 11B).

The heat treatment apparatus is not limited to an electric furnace andmay be provided with a device that heats an object to be processed bythermal conduction or thermal radiation from a heater such as aresistance heater or the like. For example, an RTA (rapid thermalanneal) apparatus such as a GRTA (gas rapid thermal anneal) apparatus oran LRTA (lamp rapid thermal anneal) apparatus can be used. An LRTAapparatus is an apparatus for heating an object to be processed byradiation of light (an electromagnetic wave) emitted from a lamp such asa halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arclamp, a high pressure sodium lamp, or a high pressure mercury lamp. AGRTA apparatus is an apparatus with which heat treatment is performedusing a high-temperature gas. As the gas, an inert gas which does notreact with a process object by heat treatment, such as nitrogen or arare gas such as argon is used.

For example, as the first heat treatment, GRTA may be performed asfollows. The substrate is transferred and put in an inert gas which hasbeen heated to a high temperature of 650° C. to 700° C., heated forseveral minutes, and transferred and taken out of the inert gas whichhas been heated to a high temperature. GRTA enables a high-temperatureheat treatment in a short time.

Note that in the first heat treatment, it is preferable that water,hydrogen, and the like be not contained in the atmosphere of nitrogen ora rare gas such as helium, neon, or argon. It is preferable that thepurity of nitrogen or the rare gas such as helium, neon, or argon whichis introduced into a heat treatment apparatus be set to be 6N (99.9999%)or higher, preferably 7N (99.99999%) or higher (that is, the impurityconcentration is 1 ppm or lower, preferably 0.1 ppm or lower).

Further, the oxide semiconductor layer may be crystallized to be amicrocrystalline film or a polycrystalline film depending on a conditionof the first heat treatment or a material of the oxide semiconductorlayer. For example, the oxide semiconductor layer may be crystallized tobe a microcrystalline semiconductor film having a degree ofcrystallization of 90% or more, or 80% or more. Further, depending onthe condition of the first heat treatment and the material of the oxidesemiconductor layer, the oxide semiconductor layer may be an amorphousoxide semiconductor film containing no crystalline component. The oxidesemiconductor layer may become an oxide semiconductor film in which amicrocrystalline portion (with a grain diameter greater than or equal to1 nm and greater than or less than 20 nm, typically greater than orequal to 2 nm and greater than or less than 4 nm) is mixed into anamorphous oxide semiconductor.

The first heat treatment for the oxide semiconductor layer can beperformed before the oxide semiconductor film 330 is processed into theisland-shaped oxide semiconductor layers. In that case, the substrate istaken out from the heat apparatus after the first heat treatment, andthen a photolithography step is performed.

The heat treatment which has an effect of dehydrating or dehydrogenatingthe oxide semiconductor layer may be performed at any of the followingtimings: after the oxide semiconductor layer is formed; after the sourceelectrode and the drain electrode are formed over the oxidesemiconductor layer; and after the protective insulating film is formedover the source electrode and the drain electrode.

In the case where a contact hole is formed in the gate insulating layer302, the formation of the contact hole may be performed before or afterthe dehydration or dehydrogenation of the oxide semiconductor film 330.

Note that the etching of the oxide semiconductor film may be dryetching, without limitation to wet etching.

The etching conditions (such as an etchant, etching time, andtemperature) are adjusted as appropriate depending on the material sothat the oxide semiconductor film can be etched into a desired shape.

Next, a conductive film is formed over the gate insulating layer 302 andthe oxide semiconductor layer 331. The conductive film may be formed bya sputtering method or a vacuum evaporation method. As examples of amaterial of the metal conductive film, an element selected from Al, Cr,Cu, Ta, Ti, Mo, or W; an alloy containing any of these elements as acomponent; an alloy film containing any of these elements incombination; and the like can be given. Alternatively, one or morematerials selected from manganese, magnesium, zirconium, beryllium, andthorium may be used. Further, the metal conductive film may have asingle-layer structure or a stacked-layer structure including two ormore layers. For example, a single-layer structure of an aluminum filmincluding silicon, a two-layer structure in which a titanium film isstacked over an aluminum film, a three-layer structure in which atitanium film, an aluminum film, and a titanium film are stacked in thisorder, and the like can be given. Alternatively, a film, an alloy film,or a nitride film which contains aluminum (Al) and one or a plurality ofelements selected from titanium (Ti), tantalum (Ta), tungsten (W),molybdenum (Mo), chromium (Cr), neodymium (Nd), and scandium (Sc) may beused.

In the case where heat treatment is performed after the formation of theconductive film, it is preferable that the conductive film have heatresistance high enough to withstand the heat treatment.

In a third photolithography step, a resist mask is formed over theconductive film, and the conductive film is selectively etched to form asource electrode layer 315 a and a drain electrode layer 315 b, and thenthe resist mask is removed (see FIG. 11C).

Ultraviolet light, KrF laser light, or ArF laser light is used for lightexposure for forming the resist mask in the third photolithography step.Channel length L of a thin film transistor to be completed later isdetermined by a distance between bottom ends of the source electrodelayer and the drain electrode layer, which are adjacent to each otherover the oxide semiconductor layer 331. Note that in the case wherelight exposure is performed so that the channel length L becomes lessthan 25 nm, light exposure for forming the resist mask in the thirdphotolithography step is performed using extreme ultraviolet with anextremely short wavelength of several nanometers to several tens ofnanometers. Light exposure using extreme ultraviolet enables highresolution and deep depth of focus. Thus, the channel length L of thethin film transistor to be completed later can be greater than or equalto 10 nm and less than or equal to 1000 nm and the operation speed of acircuit can be increased, and furthermore the value of off-state currentis extremely small, which leads to lower power consumption can beachieved.

Note that in order to prevent the oxide semiconductor layer 331 frombeing removed at the time of the etching of the conductive film,materials and etching conditions of the conductive film and the oxidesemiconductor layer 331 are adjusted as appropriate.

In this embodiment, a Ti film is used as the conductive film, anIn—Ga—Zn—O-based oxide semiconductor is used for the oxide semiconductorlayer 331, and an ammonia hydrogen peroxide solution (a mixture ofammonia, water, and a hydrogen peroxide solution) is used as an etchant.

Note that, in the third photolithography step, only part of the oxidesemiconductor layer 331 is etched to form an oxide semiconductor layerhaving a groove (a depressed portion) in some cases. In addition, theresist mask for forming the source electrode layer 315 a and the drainelectrode layer 315 b may be formed by an ink-jet method. Formation ofthe resist mask by an inkjet method needs no photomask, which results ina reduction in manufacturing costs.

Further, an oxide conductive layer may be formed between the oxidesemiconductor layer and the source and drain electrode layers. The oxideconductive layer and the metal layer for forming the source and drainelectrode layers can be formed successively. The oxide conductive layercan serve as a source and drain regions.

The provision of the oxide conductive layer as the source and drainregions between the oxide semiconductor layer and the source and drainelectrode layers makes it possible to decrease the resistance of thesource and drain regions and to operate the transistor at high speed.

In order to reduce the number of photomasks and steps in aphotolithography step, etching may be performed with the use of a resistmask formed using a multi-tone mask which is a light-exposure maskthrough which light is transmitted so as to have a plurality ofintensities. Since a resist mask formed using a multi-tone mask has aplurality of thicknesses and can be further changed in shape byperforming etching, the resist mask can be used in a plurality ofetching steps to provide different patterns. Therefore, a resist maskcorresponding to at least two kinds of different patterns can be formedby using a multi-tone mask. Thus, the number of light-exposure masks canbe reduced and the number of corresponding photolithography steps can bealso reduced, whereby simplification of a process can be realized.

Next, plasma treatment using a gas such as N₂O, N₂, or Ar is performed.This plasma treatment removes water or the like adsorbed on a surface ofthe oxide semiconductor layer which is exposed. Plasma treatment may beperformed using a mixed gas of oxygen and argon.

After the plasma treatment, an oxide insulating layer 316 which servesas a protective insulating film and is in contact with part of the oxidesemiconductor layer is formed without exposure to air.

The oxide insulating layer 316 can be formed to a thickness of at least1 nm by a method by which impurities such as water or hydrogen does notenter the oxide insulating layer 316, such as a sputtering method, asappropriate. When hydrogen is contained in the oxide insulating layer316, the hydrogen enters the oxide semiconductor layer or oxygen in theoxide semiconductor layer is extracted by the hydrogen, therebydecreasing the resistance of the oxide semiconductor layer on the sideopposite to the side where a channel is formed (so-called back channelside) (making the back channel side have n-type conductivity), whichmight result in formation of a parasitic channel. Therefore, it ispreferable that a formation method in which hydrogen is not used isemployed in order to form the oxide insulating layer 316 containing aslittle hydrogen as possible.

In this embodiment, as the oxide insulating layer 316, a silicon oxidefilm with a thickness of 200 nm is formed by a sputtering method. Thesubstrate temperature in the film formation may be higher than or equalto room temperature and lower than or equal to 300° C., and is 100° C.in this embodiment. The formation of the silicon oxide film by asputtering method can be performed in a rare gas (typically argon)atmosphere, an oxygen atmosphere, or an atmosphere of a rare gas(typically argon) and oxygen. As a target, a silicon oxide target or asilicon target can be used. For example, silicon oxide can be formedusing a silicon target by a sputtering method in an atmosphere of oxygenand nitrogen. As the oxide insulating layer 316 which is formed incontact with the oxide semiconductor layer whose resistance is reduced,an inorganic insulating film which does not include impurities such asmoisture, a hydrogen ion, and OH⁻ and blocks entry of these from theoutside is used. Typically, a silicon oxide film, a silicon nitrideoxide film, an aluminum oxide film, an aluminum oxynitride film, or thelike is used.

In that case, the oxide insulating layer 316 is preferably formed whilemoisture remaining in the treatment chamber is removed so that hydrogen,a hydroxyl group, or moisture is prevented from being contained in theoxide semiconductor layer 331 and the oxide insulating layer 316.

In order to remove moisture remaining in the treatment chamber, anentrapment vacuum pump is preferably used. For example, a cryopump, anion pump, or a titanium sublimation pump is preferably used. Theevacuation unit may be a turbo pump provided with a cold trap. In thetreatment chamber which is evacuated with the cryopump, a hydrogen atom,a compound containing a hydrogen atom such as water (H₂O), a compoundcontaining a carbon atom, and the like are removed, whereby theconcentration of an impurity in the oxide insulating layer 316 formed inthe treatment chamber can be reduced.

As a sputtering gas used for forming the oxide insulating layer 316, ahigh-purity gas is preferably used in which impurities such as hydrogen,water, a hydroxyl group, or hydride are reduced to such a level that theimpurity concentration is represented by the unit “ppm” or “ppb”.

Next, second heat treatment (preferably at higher than or equal to 200°C. and lower than or equal to 400° C., for example higher than or equalto 250° C. and lower than or equal to 350° C.) is performed in an inertgas atmosphere or an oxygen gas atmosphere. For example, the second heattreatment is performed at 250° C. in a nitrogen atmosphere for one hour.By the second heat treatment, heat is applied while part of the oxidesemiconductor layer (the channel formation region) is in contact withthe oxide insulating layer 316.

Through the above-described steps, after the heat treatment fordehydration or dehydrogenation is performed on the formed oxidesemiconductor film to reduce the resistance, part of the oxidesemiconductor film is selectively in an oxygen excess state. As aresult, a channel formation region 313 overlapping with the gateelectrode layer 311 becomes i-type, and a high-resistance source region314 a which overlaps with the source electrode layer 315 a and thehigh-resistance drain region 314 b which overlaps with the drainelectrode layer 315 b are formed in a self-aligned manner. Through theabove-described steps, the thin film transistor 310 is manufactured (seeFIG. 11D).

Furthermore, heat treatment may be performed at 100° C. to 200° C. for 1hour to 30 hours in an air atmosphere. In this embodiment, heattreatment is performed at 150° C. for 10 hours. This heat treatment maybe performed at a fixed heating temperature. Alternatively, thefollowing change in the heating temperature may be conducted pluraltimes repeatedly: the heating temperature is increased from a roomtemperature to a temperature of 100° C. to 200° C. and then decreased toa room temperature. Alternatively, this heat treatment may be performedbefore formation of the oxide insulating layer under reduced pressure.When the heat treatment is performed under reduced pressure, the heattreatment time can be shortened. This heat treatment enables anormally-off thin film transistor to be obtained. Thus, the reliabilityof a display device can be increased.

The high-resistance drain region 314 b (or the high-resistance sourceregion 314 a) is formed in a portion of the oxide semiconductor layerwhich overlaps with the drain electrode layer 315 b (or the sourceelectrode layer 315 a), whereby the reliability of the thin filmtransistor can be increased. Specifically, the formation of thehigh-resistance drain region 314 b enables a structure in which theconductivity can be gradually varied from the drain electrode layer 315b to the channel formation region 313 via the high-resistance drainregion 314 b. Thus, in the case where the transistor is operated withthe drain electrode layer 315 b connected to a wiring for supplying ahigh power supply potential Vdd, the high-resistance drain region servesas a buffer, and thus local concentration of an electric field is lesslikely to occur even if high voltage is applied between the gateelectrode layer 311 and the drain electrode layer 315 b, which leads toan increase in the withstand voltage of the transistor.

Further, the high-resistance source region or the high-resistance drainregion in the oxide semiconductor layer is formed in the entirethickness direction in the case where the thickness of the oxidesemiconductor layer is 15 nm or smaller. In the case where the thicknessof the oxide semiconductor layer is 30 nm to 50 nm, in part of the oxidesemiconductor layer, that is, in a region in the oxide semiconductorlayer which is in contact with the source electrode layer or the drainelectrode layer and the vicinity thereof, resistance is reduced. Then, ahigh-resistance source region or a high-resistance drain region isformed, while a region in the oxide semiconductor layer, which is closeto the gate insulating film, can be made to be an i-type region.

A protective insulating layer may be additionally formed over the oxideinsulating layer 316. For example, a silicon nitride film is formed byan RF sputtering method. Since an RF sputtering method has highproductivity, it is preferably used as a film formation method of theprotective insulating layer. The protective insulating layer is formedusing an inorganic insulating film which does not contain impuritiessuch as moisture, a hydrogen ion, and OH⁻ and blocks entry of these fromthe outside; for example, a silicon nitride film, an aluminum nitridefilm, a silicon nitride oxide film, an aluminum nitride oxide film, orthe like is used. In this embodiment, as the protective insulatinglayer, a protective insulating layer 303 is formed using a siliconnitride film (see FIG. 11E).

In this embodiment, as the protective insulating layer 303, a siliconnitride film is formed using a silicon semiconductor target in such amanner that the substrate 300 over which components up to and includingthe oxide insulating layer 316 are formed is heated to a temperature of100° C. to 400° C. and a sputtering gas from which hydrogen and moistureare removed and which contains high-purity nitrogen is introduced. Alsoin that case, in a manner similar to that of the oxide insulating layer316, the protective insulating layer 303 is preferably formed whilemoisture remaining in the treatment chamber is removed.

A planarization insulating layer for planarization may be provided overthe protective insulating layer 303.

As described above, the highly-purified oxide semiconductor layer isused in the thin film transistor, whereby the thin film transistor withreduced off-state current can be provided. In addition, the thin filmtransistor with reduced off-state current which is described in thisembodiment is applied to a pixel in a display device, whereby a periodin which a storage capacitor provided in the pixel can hold voltage canbe increased. Thus, a display device which consumes less power indisplaying a still image or the like can be provided.

This embodiment can be implemented in combination with any of thestructures of the other embodiments as appropriate.

(Embodiment 7)

In this embodiment, another example of a thin film transistor that canbe applied to the display device disclosed in this specification will bedescribed. A thin film transistor 360 described in this embodiment canbe used as the thin film transistor 106 in Embodiment 1.

FIGS. 12A to 12D illustrate an example of a cross-sectional structure ofthe thin film transistor of this embodiment. The thin film transistor360 illustrated in FIGS. 12A to 12D has a bottom-gate structure called achannel-protective type (channel-stop type) and is also called aninverted staggered thin film transistor.

Although the thin film transistor 360 is described as a single-gate thinfilm transistor, a multi-gate thin film transistor including a pluralityof channel formation regions can be manufactured when needed.

A process of manufacturing the thin film transistor 360 over a substrate320 will be described below with reference to FIGS. 12A to 12D.

First, a conductive film is formed over the substrate 320 having aninsulating surface, and then a first photolithography step is performedthereon to form a gate electrode layer 361. Note that a resist mask maybe formed by an inkjet method. Formation of the resist mask by an inkjetmethod needs no photomask, which results in a reduction in manufacturingcosts.

The gate electrode layer 361 can be formed to have a single-layer orstacked-layer structure including a metal material such as molybdenum,titanium, chromium, tantalum, tungsten, aluminum, copper, neodymium, orscandium, or an alloy material containing any of these materials as amain component.

Next, a gate insulating layer 322 is formed over the gate electrodelayer 361.

In this embodiment, as the gate insulating layer 322, a siliconoxynitride layer with a thickness of less than or equal to 200 nm isformed by a plasma CVD method.

Next, an oxide semiconductor film 332 is formed to a thickness greaterthan or equal to 2 nm and less than or equal to 200 nm over the gateinsulating layer 332, and then the oxide semiconductor film is processedinto an island-shaped oxide semiconductor layer in a secondphotolithography step. In this embodiment, the oxide semiconductor filmis formed by a sputtering method using an In—Ga—Zn—O-based oxidesemiconductor target for film formation.

In that case, the oxide semiconductor film is preferably formed whilemoisture remaining in a treatment chamber is removed so that hydrogen, ahydroxyl group, or moisture is prevented from being contained in theoxide semiconductor film.

In order to remove moisture remaining in the treatment chamber, anentrapment vacuum pump is preferably used. For example, a cryopump, anion pump, or a titanium sublimation pump is preferably used. Theevacuation unit may be a turbo pump provided with a cold trap. In thetreatment chamber which is evacuated with the cryopump, a hydrogen atom,a compound containing a hydrogen atom such as water (H₂O), and the likeare removed, whereby the concentration of an impurity in the oxidesemiconductor film formed in the treatment chamber can be reduced.

As a sputtering gas used for forming the oxide semiconductor film, ahigh-purity gas is preferably used in which impurities such as hydrogen,water, a hydroxyl group, or hydride are reduced to such a level that theimpurity concentration is represented by the unit “ppm” or “ppb”.

Next, dehydration or dehydrogenation of the oxide semiconductor layer isperformed. The temperature of first heat treatment for dehydration ordehydrogenation is higher than or equal to 400° C. and lower than orequal to 750° C., preferably higher than or equal to 400° C. and lowerthan the strain point of the substrate. Here, the substrate is put in anelectric furnace that is a kind of heat treatment apparatus and heattreatment is performed on the oxide semiconductor layer in a nitrogenatmosphere at 450° C. for one hour, and then water and hydrogen areprevented from entering the oxide semiconductor layer with the oxidesemiconductor layer not exposed to air. In the above manner, an oxidesemiconductor layer 332 is obtained (see FIG. 12A).

Next, plasma treatment using a gas such as N₂O, N₂, or Ar is performed.This plasma treatment removes water or the like adsorbed on a surface ofthe oxide semiconductor layer which is exposed. Plasma treatment may beperformed using a mixed gas of oxygen and argon.

Next, an oxide insulating layer is formed over the gate insulating layer322 and the oxide semiconductor layer 332. After that, in a thirdphotolithography step, a resist mask is formed, and the oxide insulatinglayer is selectively etched to form an oxide insulating layer 366, andthen, the resist mask is removed.

In this embodiment, as the oxide insulating layer 366, a silicon oxidefilm with a thickness of 200 nm is formed by a sputtering method. Thesubstrate temperature in the film formation may be higher than or equalto room temperature and lower than or equal to 300° C., and is 100° C.in this embodiment. The formation of the silicon oxide film by asputtering method can be performed in a rare gas (typically argon)atmosphere, an oxygen atmosphere, or an atmosphere of a rare gas(typically argon) and oxygen. As a target, a silicon oxide target or asilicon target can be used. For example, silicon oxide can be formedusing a silicon target by a sputtering method in an atmosphere of oxygenand nitrogen. As the oxide insulating layer 366 which is formed incontact with the oxide semiconductor layer whose resistance is reduced,an inorganic insulating film which does not include impurities such asmoisture, a hydrogen ion, and OH⁻ and blocks entry of these from theoutside is used. Typically, a silicon oxide film, a silicon nitrideoxide film, an aluminum oxide film, an aluminum oxynitride film, or thelike is used.

In that case, the oxide insulating layer 366 is preferably formed whilemoisture remaining in the treatment chamber is removed so that hydrogen,a hydroxyl group, or moisture is prevented from being contained in theoxide semiconductor layer 332 and the oxide insulating layer 366.

In order to remove moisture remaining in the treatment chamber, anentrapment vacuum pump is preferably used. For example, a cryopump, anion pump, or a titanium sublimation pump is preferably used. Theevacuation unit may be a turbo pump provided with a cold trap. In thetreatment chamber which is evacuated with the cryopump, a hydrogen atom,a compound containing a hydrogen atom such as water (H₂O), a compoundcontaining a carbon atom, and the like are removed, whereby theconcentration of an impurity in the oxide insulating layer 366 formed inthe treatment chamber can be reduced.

As a sputtering gas used for forming the insulating layer 366, ahigh-purity gas is preferably used in which impurities such as hydrogen,water, a hydroxyl group, or hydride are reduced to such a level that theimpurity concentration is represented by the unit “ppm” or “ppb”.

Next, second heat treatment (preferably at higher than or equal to 200°C. and lower than or equal to 400° C., for example higher than or equalto 250° C. and lower than or equal to 350° C.) may be performed in aninert gas atmosphere or an oxygen gas atmosphere. For example, thesecond heat treatment is performed at 250° C. in a nitrogen atmospherefor one hour. By the second heat treatment, heat is applied while partof the oxide semiconductor layer (the channel formation region) is incontact with the oxide insulating layer 366.

In this embodiment, the oxide semiconductor layer 332 which is providedwith the oxide insulating layer 366 and is partly exposed is furthersubjected to heat treatment in a nitrogen atmosphere or an inert gasatmosphere or under reduced pressure. By the heat treatment in anitrogen atmosphere or an inert gas atmosphere or under reducedpressure, the resistance of the exposed region of the oxidesemiconductor layer 441, which is not covered with the oxide insulatinglayer 366, can be reduced. For example, the heat treatment is performedat 250° C. in a nitrogen atmosphere for one hour.

By the heat treatment performed on the oxide semiconductor layer 332provided with the oxide insulating layer 366 in a nitrogen atmosphere,the resistance of the exposed region of the oxide semiconductor layer332 is reduced, whereby an oxide semiconductor layer 362 includingregions with different resistances (indicated as a shaded region and awhite region in FIG. 12B) is formed.

Next, a conductive film is formed over the gate insulating layer 322,the oxide semiconductor layer 362, and the oxide insulating layer 366.After that, in a fourth photolithography step, the conductive film isselectively etched to form a source electrode layer 365 a and a drainelectrode layer 365 b, and then the resist mask is removed (see FIG.12C).

As the material of the source electrode layer 365 a and the drainelectrode layer 365 b, there are an element selected from Al, Cr, Cu,Ta, Ti, Mo, and W, an alloy containing any of these elements as itscomponent, an alloy containing a combination of any of these elements,and the like. Further, the conductive film may have a single-layerstructure or a stacked-layer structure including two or more layers.

Through the above-described steps, after the heat treatment fordehydration or dehydrogenation is performed on the formed oxidesemiconductor film to reduce the resistance, part of the oxidesemiconductor film is selectively in an oxygen excess state. As aresult, a channel formation region 363 overlapping with the oxideinsulating layer 366 becomes i-type, and a high-resistance source region364 a which overlaps with the source electrode layer 365 a and thehigh-resistance drain region 364 b which overlaps with the drainelectrode layer 365 b are formed in a self-aligned manner. Through theabove-described steps, the thin film transistor 360 is manufactured.

Furthermore, heat treatment may be performed at 100° C. to 200° C. for 1hour to 30 hours in an air atmosphere. In this embodiment, heattreatment is performed at 150° C. for 10 hours. This heat treatment maybe performed at a fixed heating temperature. Alternatively, thefollowing change in the heating temperature may be conducted pluraltimes repeatedly: the heating temperature is increased from a roomtemperature to a temperature of 100° C. to 200° C. and then decreased toa room temperature. Alternatively, this heat treatment may be performedbefore formation of the oxide insulating layer under reduced pressure.When the heat treatment is performed under reduced pressure, the heattreatment time can be shortened. This heat treatment enables anormally-off thin film transistor. Thus, the reliability can beincreased.

The high-resistance drain region 364 b (or the high-resistance sourceregion 364 a) is formed in a portion of the oxide semiconductor layerwhich overlaps with the drain electrode layer 365 b (or the sourceelectrode layer 365 a), whereby the reliability of the thin filmtransistor can be increased. Specifically, the formation of thehigh-resistance drain region 364 b enables a structure in which theconductivity can be gradually varied from the drain electrode layer 365b to the channel formation region 363 via the high-resistance drainregion 364 b. Thus, in the case where the transistor is operated withthe drain electrode layer 365 b connected to a wiring for supplying ahigh power supply potential Vdd, the high-resistance drain region 364 bserves as a buffer, and thus local concentration of an electric field isless likely to occur even if high voltage is applied between the gateelectrode layer 361 and the drain electrode layer 365 b, which leads toan increase in the withstand voltage of the transistor.

Next, a protective insulating layer 323 is formed over the sourceelectrode layer 365 a, the drain electrode layer 365 b, and the oxideinsulating layer 366. In this embodiment, the protective insulatinglayer 323 is formed using a silicon nitride film (see FIG. 12D).

Note that an oxide insulating layer may be additionally formed over thesource electrode layer 365 a, the drain electrode layer 365 b, and theoxide insulating layer 366, and a protective insulating layer 323 may bestacked over the oxide insulating layer.

As described above, the highly-purified oxide semiconductor layer isused in the thin film transistor, whereby the thin film transistor withreduced off-state current can be provided. In addition, the thin filmtransistor with reduced off-state current which is described in thisembodiment is applied to a pixel in a display device, whereby a periodin which a storage capacitor provided in the pixel can hold voltage canbe increased. Thus, a display device which consumes less power indisplaying a still image or the like can be provided.

This embodiment can be implemented in combination with any of thestructures of the other embodiments as appropriate.

(Embodiment 8)

In this embodiment, another example of a thin film transistor that canbe applied to the display device disclosed in this specification will bedescribed. A thin film transistor 350 described in this embodiment canbe used as the thin film transistor 106 in Embodiment 1.

FIGS. 13A to 13D illustrate an example of a cross-sectional structure ofthe thin film transistor of this embodiment.

Although the thin film transistor 350 is described as a single-gate thinfilm transistor, a multi-gate thin film transistor including a pluralityof channel formation regions can be manufactured when needed.

A process of manufacturing the thin film transistor 350 over a substrate340 will be described below with reference to FIGS. 13A to 13D.

First, a conductive film is formed over the substrate 340 having aninsulating surface, and then a first photolithography step is performedthereon to form a gate electrode layer 351. In this embodiment, as thegate electrode layer 351, a tungsten film with a thickness of 150 nm isformed by a sputtering method.

Next, a gate insulating layer 342 is formed over the gate electrodelayer 351. In this embodiment, as the gate insulating layer 342, asilicon oxynitride layer with a thickness of less than or equal to 100nm is formed by a plasma CVD method.

Next, a conductive film is formed over the gate insulating layer 342.After that, in a second photolithography step, a resist mask is formedover the conductive film, and the conductive film is selectively etchedto form a source electrode layer 355 a and a drain electrode layer 355b, and then the resist mask is removed (see FIG. 13A).

Next, an oxide semiconductor film 345 is formed (see FIG. 13B). In thisembodiment, as the oxide semiconductor film 345 is formed using anIn—Ga—Zn—O-based oxide semiconductor target for film formation by asputtering method. In a third photolithography step, the oxidesemiconductor film 345 is processed into an island-shaped oxidesemiconductor layer.

In that case, the oxide semiconductor film 345 is preferably formedwhile moisture remaining in a treatment chamber is removed so thathydrogen, a hydroxyl group, or moisture is prevented from beingcontained in the oxide semiconductor film 345.

In order to remove moisture remaining in the treatment chamber, anentrapment vacuum pump is preferably used. For example, a cryopump, anion pump, or a titanium sublimation pump is preferably used. Theevacuation unit may be a turbo pump provided with a cold trap. In thetreatment chamber which is evacuated with the cryopump, a hydrogen atom,a compound containing a hydrogen atom such as water (H₂O), and the likeare removed, whereby the concentration of an impurity in the oxidesemiconductor film 345 formed in the treatment chamber can be reduced.

As a sputtering gas used for forming the oxide semiconductor film 345, ahigh-purity gas is preferably used in which impurities such as hydrogen,water, a hydroxyl group, or hydride are reduced to such a level that theimpurity concentration is represented by the unit “ppm” or “ppb”.

Next, dehydration or dehydrogenation of the oxide semiconductor layer isperformed. The temperature of first heat treatment for dehydration ordehydrogenation is higher than or equal to 400° C. and lower than orequal to 750° C., preferably higher than or equal to 400° C. and lowerthan the strain point of the substrate. Here, the substrate is put in anelectric furnace that is a kind of heat treatment apparatus and heattreatment is performed on the oxide semiconductor layer in a nitrogenatmosphere at 450° C. for one hour, and then water and hydrogen areprevented from entering the oxide semiconductor layer with the oxidesemiconductor layer not exposed to air. In the above manner, an oxidesemiconductor layer 346 is obtained (see FIG. 13C).

As the first heat treatment, GRTA may be performed as follows. Thesubstrate is transferred and put in an inert gas which has been heatedto a high temperature of 650° C. to 700° C., heated for several minutes,and transferred and taken out of the inert gas which has been heated toa high temperature. GRTA enables a high-temperature heat treatment in ashort time.

Next, an oxide insulating layer 356 serving as a protective insulatingfilm is formed in contact with the oxide semiconductor layer 346.

The oxide insulating layer 356 can be formed to a thickness of at least1 nm by a method by which impurities such as water or hydrogen does notenter the oxide insulating layer 356, such as a sputtering method, asappropriate. When hydrogen is contained in the oxide insulating layer356, the hydrogen enters the oxide semiconductor layer or oxygen in theoxide semiconductor layer is extracted by the hydrogen, therebydecreasing the resistance of the oxide semiconductor layer on the sideopposite to the side where a channel is formed (the so-called backchannel side) (making the back channel side have n-type conductivity),which might result in formation of a parasitic channel. Therefore, it ispreferable that a formation method in which hydrogen is not used isemployed in order to form the oxide insulating layer 356 containing aslittle hydrogen as possible.

In this embodiment, as the oxide insulating layer 356, a silicon oxidefilm with a thickness of 200 nm is formed by a sputtering method. Thesubstrate temperature in the film formation may be higher than or equalto room temperature and lower than or equal to 300° C., and is 100° C.in this embodiment. The formation of the silicon oxide film by asputtering method can be performed in a rare gas (typically argon)atmosphere, an oxygen atmosphere, or an atmosphere of a rare gas(typically argon) and oxygen. As a target, a silicon oxide target or asilicon target can be used. For example, silicon oxide can be formedusing a silicon target by a sputtering method in an atmosphere of oxygenand nitrogen. As the oxide insulating layer 356 which is formed incontact with the oxide semiconductor layer whose resistance is reduced,an inorganic insulating film which does not include impurities such asmoisture, a hydrogen ion, and OH⁻ and blocks entry of these from theoutside is used. Typically, a silicon oxide film, a silicon nitrideoxide film, an aluminum oxide film, an aluminum oxynitride film, or thelike is used.

In that case, the oxide insulating layer 356 is preferably formed whilemoisture remaining in the treatment chamber is removed so that hydrogen,a hydroxyl group, or moisture is prevented from being contained in theoxide semiconductor layer 346 and the oxide insulating layer 356.

In order to remove moisture remaining in the treatment chamber, anentrapment vacuum pump is preferably used. For example, a cryopump, anion pump, or a titanium sublimation pump is preferably used. Theevacuation unit may be a turbo pump provided with a cold trap. In thetreatment chamber which is evacuated with the cryopump, a hydrogen atom,a compound containing a hydrogen atom such as water (H₂O), a compoundcontaining a carbon atom, and the like are removed, whereby theconcentration of an impurity in the oxide insulating layer 356 formed inthe treatment chamber can be reduced.

As a sputtering gas used for forming the oxide insulating layer 356, ahigh-purity gas is preferably used in which impurities such as hydrogen,water, a hydroxyl group, or hydride are reduced to such a level that theimpurity concentration is represented by the unit “ppm” or “ppb”.

Next, second heat treatment (preferably at higher than or equal to 200°C. and lower than or equal to 400° C., for example higher than or equalto 250° C. and lower than or equal to 350° C.) is performed in an inertgas atmosphere or an oxygen gas atmosphere. For example, the second heattreatment is performed at 250° C. in a nitrogen atmosphere for one hour.By the second heat treatment, heat is applied while the oxidesemiconductor layer is in contact with the oxide insulating layer 356.

Through the above-described steps, after the heat treatment fordehydration or dehydrogenation is performed on the formed oxidesemiconductor film to reduce the resistance, the oxide semiconductorfilm is in an oxygen excess state. As a result, an i-type oxidesemiconductor layer 352 is formed. Through the above-described steps,the thin film transistor 350 is manufactured.

Furthermore, heat treatment may be performed at 100° C. to 200° C. for 1hour to 30 hours in an air atmosphere. In this embodiment, heattreatment is performed at 150° C. for 10 hours. This heat treatment maybe performed at a fixed heating temperature. Alternatively, thefollowing change in the heating temperature may be conducted pluraltimes repeatedly: the heating temperature is increased from a roomtemperature to a temperature of 100° C. to 200° C. and then decreased toa room temperature. Alternatively, this heat treatment may be performedbefore formation of the oxide insulating layer under reduced pressure.When the heat treatment is performed under reduced pressure, the heattreatment time can be shortened. This heat treatment enables anormally-off thin film transistor. Thus, the reliability can beincreased.

A protective insulating layer may be additionally formed over the oxideinsulating layer 356. For example, a silicon nitride film is formed byan RF sputtering method. In this embodiment, as the protectiveinsulating layer, a protective insulating layer 343 is formed using asilicon nitride film (see FIG. 13D).

A planarization insulating layer for planarization may be provided overthe protective insulating layer 343.

As described above, the highly-purified oxide semiconductor layer isused in the thin film transistor, whereby the thin film transistor withreduced off-state current can be provided. In addition, the thin filmtransistor with reduced off-state current which is described in thisembodiment is applied to a pixel in a display device, whereby a periodin which a storage capacitor provided in the pixel can hold voltage canbe increased. Thus, a display device which consumes less power indisplaying a still image or the like can be provided.

This embodiment can be implemented in combination with any of thestructures of the other embodiments as appropriate.

(Embodiment 9)

In this embodiment, another example of a thin film transistor that canbe applied to the display device disclosed in this specification will bedescribed. A thin film transistor 380 described in this embodiment canbe used as the thin film transistor 106 in Embodiment 1.

In this embodiment, an example of a manufacturing process of a thin filmtransistor, which is partly different from that of Embodiment 6, will bedescribed with reference to FIG. 14. Since a manufacturing process of athin film transistor in FIG. 14 is the same as that of the thin filmtransistor in FIGS. 11A to 11E except for some steps, the same referencenumerals are used for the same portions, and detailed description of thesame portions is not repeated.

In accordance with Embodiment 6, a gate electrode layer 381 is formedover a substrate 370, and a first gate insulating layer 372 a and asecond gate insulating layer 372 b are stacked thereover. In thisembodiment, the gate insulating layer has a two-layer structure: anitride insulating layer is used as the first gate insulating layer 372a, and an oxide insulating layer is used as the second gate insulatinglayer 372 b.

As the oxide insulating layer, a silicon oxide layer, a siliconoxynitride layer, an aluminum oxide layer, an aluminum oxynitride layer,or the like can be used. As the nitride insulating layer, a siliconnitride layer, a silicon nitride oxide layer, an aluminum nitride layer,an aluminum nitride oxide layer, or the like can be used.

In this embodiment, the insulating layer has a structure in which asilicon nitride layer and a silicon oxide layer are stacked in thisorder over the gate electrode layer 381. A silicon nitride layer(SiN_(y) (y>0) with a thickness of greater than or equal to 50 nm andless than or equal to 200 nm (50 nm in this embodiment) is formed as thefirst gate insulating layer 372 a by a sputtering method, and a siliconoxide layer (SiO_(x) (x>0) with a thickness of greater than or equal to5 nm and less than or equal to 300 nm (100 nm in this embodiment) isstacked as the second gate insulating layer 372 b over the first gateinsulating layer 372 a, whereby a gate insulating layer with a thicknessof 150 nm is formed.

Next, an oxide semiconductor film is formed, and is processed into anisland-shape oxide semiconductor layer in a photolithography step. Inthis embodiment, the oxide semiconductor film is formed using anIn—Ga—Zn—O-based oxide semiconductor target for film formation by asputtering method.

In that case, the oxide semiconductor film is preferably formed whilemoisture remaining in the treatment chamber is removed so that hydrogen,a hydroxyl group, or moisture is prevented from being contained in theoxide semiconductor film.

In order to remove moisture remaining in the treatment chamber, anentrapment vacuum pump is preferably used. For example, a cryopump, anion pump, or a titanium sublimation pump is preferably used. Theevacuation unit may be a turbo pump provided with a cold trap. In thetreatment chamber which is evacuated with the cryopump, a hydrogen atom,a compound containing a hydrogen atom such as water (H₂O), and the likeare removed, whereby the concentration of an impurity in the oxidesemiconductor film formed in the treatment chamber can be reduced.

As a sputtering gas used for forming the oxide semiconductor film, ahigh-purity gas is preferably used in which impurities such as hydrogen,water, a hydroxyl group, or hydride are reduced to such a level that theimpurity concentration is represented by the unit “ppm” or “ppb”.

Next, dehydration or dehydrogenation of the oxide semiconductor layersis performed. The temperature of the first heat treatment fordehydration or dehydrogenation is higher than or equal to 400° C. andlower than or equal to 750° C., preferably higher than or equal to 425°C. Note that in the case where the temperature is higher than or equalto 425° C., the heat treatment time may be one hour or less, whereas inthe case where the temperature is lower than 425° C., the heat treatmenttime is longer than one hour. Here, the substrate is introduced into anelectric furnace, that is a kind of heat treatment apparatus, and heattreatment is performed on the oxide semiconductor layers in a nitrogenatmosphere, and then water and hydrogen are prevented from entering theoxide semiconductor layer with the oxide semiconductor layer not exposedto air. In the above manner, the oxide semiconductor layer is obtained.After that, cooling is performed by introduction of a high-purity oxygengas, a high-purity N₂O gas, or ultra-dry air (having a dew point of −40°C. or lower, preferably −60° C. or lower) into the same furnace. It ispreferable that the oxygen gas and the N₂O gas do not include water,hydrogen, and the like. Alternatively, the purity of an oxygen gas or anN₂O gas which is introduced into the heat treatment apparatus ispreferably 6N (99.9999%) or higher, more preferably 7N (99.99999%) orhigher (that is, the impurity concentration of the oxygen gas or the N₂Ogas is 1 ppm or lower, preferably 0.1 ppm or lower).

The heat treatment apparatus is not limited to the electric furnace, andmay be, for example, an RTA (rapid thermal annealing) apparatus such asa GRTA (gas rapid thermal annealing) apparatus or an LRTA (lamp rapidthermal annealing) apparatus. An LRTA apparatus is an apparatus forheating an object to be processed by radiation of light (anelectromagnetic wave) emitted from a lamp such as a halogen lamp, ametal halide lamp, a xenon arc lamp, a carbon arc lamp, a high pressuresodium lamp, or a high pressure mercury lamp. In addition, the LRTAapparatus may be provided with not only a lamp but also a device whichheats an object to be processed by heat conduction or heat radiationfrom a heater such as a resistance heater. GRTA is a method of heattreatment using a high-temperature gas. As the gas, an inert gas whichdoes not react with a process object by heat treatment, such as nitrogenor a rare gas such as argon is used. The heat treatment may be performedat 600° C. to 750° C. for several minutes using an RTA method.

Further, after the first heat treatment for dehydration ordehydrogenation, heat treatment may be performed at higher than or equalto 200° C. and lower than or equal to 400° C., preferably higher than orequal to 200° C. and lower than or equal to 300° C., in an atmosphere ofan oxygen gas or an N₂O gas.

The first heat treatment for the oxide semiconductor layer can beperformed before the oxide semiconductor film is processed into theisland-shaped oxide semiconductor layer. In that case, the substrate istaken out of the heat apparatus after the first heat treatment, and thena photolithography step is performed.

Through the above-described steps, the entire oxide semiconductor filmis in an oxygen-excess state, whereby the oxide semiconductor film hashigh resistance, that is, the oxide semiconductor film has i-typeconductivity. Thus, an oxide semiconductor layer 382 the entire regionof which has i-type conductivity is obtained.

Next, a conductive film is formed over the gate insulating layer 372 band the oxide semiconductor layer 382. Furthermore, in aphotolithography step, a resist mask is formed over the conductive film,and the conductive film is selectively etched to form a source electrodelayer 385 a and a drain electrode layer 385 b, and then, an oxideinsulating layer 386 is formed by a sputtering method.

In that case, the oxide insulating layer 386 is preferably formed whilemoisture remaining in the treatment chamber is removed so that hydrogen,a hydroxyl group, or moisture is prevented from being contained in theoxide semiconductor layer 382 and the oxide insulating layer 386.

In order to remove moisture remaining in the treatment chamber, anentrapment vacuum pump is preferably used. For example, a cryopump, anion pump, or a titanium sublimation pump is preferably used. Theevacuation unit may be a turbo pump provided with a cold trap. In thetreatment chamber which is evacuated with the cryopump, a hydrogen atom,a compound containing a hydrogen atom such as water (H₂O), and the likeare removed, whereby the concentration of an impurity in the oxideinsulating layer 386 formed in the treatment chamber can be reduced.

As a sputtering gas used for forming the oxide insulating layer 386, ahigh-purity gas is preferably used in which impurities such as hydrogen,water, a hydroxyl group, or hydride are reduced to such a level that theimpurity concentration is represented by the unit “ppm” or “ppb”.

Through the above-described steps, the thin film transistor 380 can bemanufactured.

Next, in order to reduce variation in electric characteristics of thethin film transistor, heat treatment (preferably at higher than or equalto 150° C. and lower than 350° C.) may be performed in an inert gasatmosphere or a nitrogen gas atmosphere. For example, the heat treatmentis performed at 250° C. in a nitrogen atmosphere for 1 hour.

The heat treatment may be performed at higher than or equal to 100° C.and lower than or equal to 200° C. in an air atmosphere for 1 hour to 30hours. In this embodiment, the heat treatment is performed at 150° C.for 10 hours. This heat treatment may be performed at a fixed heatingtemperature. Alternatively, the following change in the heatingtemperature may be conducted plural times repeatedly: the heatingtemperature is increased from a room temperature to a temperature of100° C. to 200° C. and then decreased to a room temperature.Alternatively, this heat treatment may be performed before formation ofthe oxide insulating film under reduced pressure. When the heattreatment is performed under reduced pressure, the heat treatment timecan be shortened. This heat treatment enables a normally-off thin filmtransistor. Thus, the reliability can be increased.

Next, a protective insulating layer 373 is formed over the oxideinsulating layer 386. In this embodiment, as the protective insulatinglayer 373, a silicon nitride film with a thickness of 100 nm is formedby a sputtering method.

The protective insulating layer 373 and the first gate insulating layer372 a which are formed of a nitride insulating layer do not containimpurities such as moisture hydrogen, hydride, and hydroxide and have aneffect of blocking entry of the impurities from the outside.

Thus, entry of impurities such as moisture from the outside can beprevented in the manufacturing process after the formation of theprotective insulating layer 373. Moreover, entry of moisture from theoutside can be prevented in the long term even after a device iscompleted as a semiconductor device; thus, the long-term reliability ofthe device can be achieved.

Alternatively, a structure may be employed in which the insulatinglayers provided between the protective insulating layer 373 and thefirst gate insulating layer 372 a which are formed of a nitrideinsulating layer are removed so that the protective insulating layer 373is in contact with the first gate insulating layer 372 a.

Thus, impurities such as moisture, hydrogen, hydride, and hydroxide canbe reduced to a minimum, reentry of the impurities can be prevented, andthe concentration of impurities in the oxide semiconductor layer can bemaintained low.

Note that a planarization insulating layer for planarization may beprovided over the protective insulating layer 373.

As described above, the highly-purified oxide semiconductor layer isused in the thin film transistor, whereby the thin film transistor withreduced off-state current can be provided. In addition, the thin filmtransistor with reduced off-state current which is described in thisembodiment is applied to a pixel in a display device, whereby a periodin which a storage capacitor provided in the pixel can hold voltage canbe increased. Thus, a display device which consumes less power indisplaying a still image or the like can be provided.

This embodiment can be implemented in combination with any of thestructures of the other embodiments as appropriate.

(Embodiment 10)

In this embodiment, another example of a thin film transistor that canbe applied to the display device disclosed in this specification will bedescribed. The thin film transistor described in this embodiment can beused as the thin film transistors in Embodiments 2 to 8.

In this embodiment, an example in which light-transmitting conductivematerials are used for a gate electrode layer, a source electrode layer,and a drain electrode layer will be described. Other than the above, thethin film transistor can be manufactured in a manner similar to those ofthe above embodiments, and description of the same parts or parts havingfunctions and process similar to those in the above embodiments is notrepeated. In addition, detailed description of the same parts isomitted.

As materials of the gate electrode layer, the source electrode layer,and the drain electrode layer, a conductive material that transmitsvisible light can be used. For example, any of the following metaloxides can be used: an In—Sn—O-based metal oxide; an In—Sn—Zn—O-basedmetal oxide; an In—Al—Zn—O-based metal oxide; a Sn—Ga—Zn—O-based metaloxide; an Al—Ga—Zn—O-based metal oxide; a Sn—Al—Zn—O-based metal oxide;an In—Zn—O-based metal oxide; a Sn—Zn—O-based metal oxide; anAl—Zn—O-based metal oxide; an In—O-based metal oxide; a Sn—O-based metaloxide; and a Zn—O-based metal oxide. The thickness thereof is set in therange of greater than or equal to 50 nm and less than or equal to 300 nmas appropriate. As a deposition method of the metal oxide used for thegate electrode layer, the source electrode layer, and the drainelectrode layer, a sputtering method, a vacuum evaporation method (anelectron beam evaporation method or the like), an arc discharge ionplating method or a spray method is used. When a sputtering method isemployed, it is preferable that deposition be performed using a targetcontaining SiO₂ of greater than or equal to 2 wt % and less than orequal to 10 wt % and SiO_(x) (x>0) which inhibits crystallization becontained in the light-transmitting conductive film so as to preventcrystallization of the oxide conductive film at the time of the heattreatment performed later.

Note that the unit of the percentage of components in thelight-transmitting conductive film is atomic percent, and the percentageof components is evaluated by analysis using an electron probe X-raymicroanalyzer (EPMA).

In a pixel in which the thin film transistor is provided, when a pixelelectrode layer, another electrode layer (such as a capacitor electrodelayer), or a wiring layer (such as a capacitor wiring layer) is formedusing a conductive film that transmits visible light, a display devicehaving high aperture ratio can be realized. Needless to say, it ispreferable that each of a gate insulating layer, an oxide insulatinglayer, a protective insulating layer, and a planarization insulatinglayer in the pixel be also formed using a film that transmits visiblelight.

In this specification, a film that transmits visible light means a filmhaving such a thickness as to have transmittance of visible light of 75%to 100%. In the case where the film has conductivity, the film is alsoreferred to as a transparent conductive film. Further, a conductive filmwhich is semi-transmissive with respect to visible light may be used formetal oxide applied to the gate electrode layer, the source electrodelayer, the drain electrode layer, the pixel electrode layer, anotherelectrode layer, or another wiring layer. The conductive film which issemi-transmissive with respect to visible light indicates a film havingtransmittance of visible light of 50% to 75%.

When a thin film transistor has a light-transmitting property asdescribed above, the aperture ratio can be increased. For small liquidcrystal display panels of 10 inches or smaller in particular, highaperture ratio can be achieved even when the size of pixels is decreasedin order to realize higher resolution of display images by, for example,increasing the number of gate wirings. Further, by using alight-transmitting film for components in a thin film transistor, evenwhen a group of high-density thin film transistors is provided, highaperture ratio can be obtained and a sufficient area of a display regioncan be secured. Further, a storage capacitor may be formed using thesame material by the same step as the component in the thin filmtransistor so that the storage capacitor can have light-transmittingproperties, by which the aperture ratio can be further improved.

As described above, the highly-purified oxide semiconductor layer isused in the thin film transistor, whereby the thin film transistor withreduced off-state current can be provided. In addition, the thin filmtransistor with reduced off-state current which is described in thisembodiment is applied to a pixel in a display device, whereby a periodin which a storage capacitor provided in the pixel can hold voltage canbe increased. Thus, a display device which consumes less power indisplaying a still image or the like can be provided.

This embodiment can be implemented in combination with any of thestructures of the other embodiments as appropriate.

(Embodiment 11)

In this embodiment, an example of a light-emitting element that can beapplied to the display device disclosed in this specification will bedescribed with reference to FIG. 15, FIGS. 16A to 16C, and FIGS. 17A and17B.

In this embodiment, as a display element included in a pixel of adisplay device, a light-emitting element utilizing electroluminescencewill be described as an example. Light-emitting elements utilizingelectroluminescence are classified according to whether a light-emittingmaterial is an organic compound or an inorganic compound. In general,the former is referred to as an organic EL element, and the latter isreferred to as an inorganic EL element.

The organic EL element includes an anode, a cathode, and a layercontaining an organic compound between the anode and the cathode. Thepotential of the anode is made to be higher than that of the cathode sothat holes are injected from the anode and electrons are injected fromthe cathode to the layer containing an organic compound. When theelectrons and holes (carriers) are recombined in the layer containing anorganic compound, energy is generated, the organic compound havinglight-transmitting properties is excited by the generated energy, andlight is emitted when the excited organic compound returns to a groundstate. Because of such a mechanism, the organic EL element is an exampleof a current-excitation light-emitting element.

The inorganic EL elements are classified according to their elementstructures into a dispersion-type inorganic EL element and a thin-filminorganic EL element. A dispersion-type inorganic EL element has alight-emitting layer where particles of a light-emitting material aredispersed in a binder, and its light emission mechanism isdonor-acceptor recombination type light emission that utilizes a donorlevel and an acceptor level. A thin-film inorganic EL element has astructure where a light-emitting layer is sandwiched between dielectriclayers, which are further sandwiched between electrodes, and its lightemission mechanism is localized type light emission that utilizesinner-shell electron transition of metal ions. Note that description ismade here using an organic EL element as a light-emitting element.

FIG. 15 is an equivalent circuit diagram of an example of a pixelconfiguration.

The configuration and operation of a pixel will be described. Here, onepixel includes two n-channel transistors in each of which an oxidesemiconductor layer is used in a channel formation region.

A pixel 6400 includes a switching transistor 6401, a driving transistor6402, and a light-emitting element 6404. A gate of the switchingtransistor 6401 is connected to a scan line 6406, a first electrode (oneof a source electrode and a drain electrode) of the switching transistor6401 is connected to a signal line 6405, and a second electrode (theother of the source electrode and the drain electrode) of the switchingtransistor 6401 is connected to a gate of the driver transistor 6402. Agate of the driving transistor 6402 is connected to the second electrode(the other of the source electrode and the drain electrode) of theswitching transistor 6401, the first electrode is connected to a powersupply line 6407, and the second electrode is connected to a firstelectrode (a pixel electrode) of the light-emitting element 6404. Asecond electrode of the light-emitting element 6404 corresponds to acommon electrode 6408. The common electrode 6408 is electricallyconnected to a common potential line provided over the same substrate.

The second electrode (the common electrode 6408) of the light-emittingelement 6404 is set to a low power supply potential. Note that the lowpower supply potential is a potential smaller than a high power supplypotential when the high power supply potential set to the power supplyline 6407 is a reference. As the low power supply potential, GND, 0 V,or the like may be adopted, for example. A potential difference betweenthe high power supply potential and the low power supply potential isapplied to the light-emitting element 6404 and current is supplied tothe light-emitting element 6404, so that the light-emitting element 6404emits light. Here, in order to make the light-emitting element 6404 emitlight, each potential is set so that the potential difference betweenthe high power supply potential and the low power supply potential is aforward threshold voltage or higher.

Further, DC power is supplied to the power supply line 6407. Inparticular, pulsed DC power is supplied to the power supply line 6407 sothat the light-emitting element 6404 can emit pulsed light. Bydisplaying pulsed images at intervals, a plurality of still images canbe switched and displayed in order. Such a display can be used for adisplay of a clock or a watch that ticks away the seconds. Note that DCpower without voltage change may be supplied.

In this embodiment, a thin film transistor in which a highly-purifiedoxide semiconductor layer is included and off-state current issuppressed is used in the pixel portion; thus, potential which iswritten to the gate of the driving transistor 6402 is held while theswitching transistor 6401 is off. Note that a capacitor may be providedbetween the gate of the driving transistor 6402 and the power supplyline 6407.

As an example of a driving method of the light-emitting element 6404, amethod of performing analog gray scale driving will be described.Voltage higher than or equal to voltage which is the sum of the forwardvoltage of the light-emitting element 6404 and V_(th) of the drivingtransistor 6402 is applied to the gate of the driving transistor 6402.The forward voltage of the light-emitting element 6404 indicates avoltage at which a desired luminance is obtained, and includes at leastforward threshold voltage. By inputting a video signal to enable thedriving transistor 6402 to operate in a saturation region, current canbe supplied to the light-emitting element 6404. In order for the drivertransistor 6402 to operate in the saturation region, the potential ofthe power supply line 6407 is set higher than the gate potential of thedriver transistor 6402. When an analog video signal is used, it ispossible to feed current to the light-emitting element 6404 inaccordance with the video signal and perform analog grayscale driving.

Note that an oxide semiconductor layer provided over a substrate hasless variation in its characteristics. Therefore, a plurality of thinfilm transistors each including the oxide semiconductor layer which arearranged in a display region also have uniform characteristics. Thedriving transistor 6402 has less variation in the characteristics andcan be precisely controlled in accordance with gate voltage to whichcurrent flowing to the light-emitting element 6404 is written. As aresult, the display device that is one embodiment of the presentinvention is capable of high-quality display with less displayunevenness.

Further, a voltage-input voltage-driving method enables an areagrayscale display using a plurality of pixels, color display (e.g., R+G,G+B, R+B, and R+G+B) by combination of a plurality of pixels (R, G, andB) having different emission colors, and the like. In the case of thevoltage-input voltage-driving method, a signal is input to the gate ofthe driving transistor 6402 so that the driving transistor 6402 iscompletely turned on or off. In other words, the driving transistor 6402operates in a linear region. Since the driving transistor 6402 operatesin the linear region, voltage higher than the voltage of the powersupply line 6407 is applied to the gate of the driving transistor 6402.Note that voltage higher than or equal to voltage which is the sum ofthe voltage of the power supply line and V_(th) of the drivingtransistor 6402 is applied to the signal line 6405.

Note that also in the case where the light-emitting element 6404 isdriven by an analog grayscale driving method or a voltage-inputvoltage-driving method, the gate potential of the driving transistor6402 can be held for a long time because off-state current of theswitching transistor 6401 per a channel width of 1 μm is suppressed to,for example, lower than or equal to 1×10⁻¹⁶ A/μm. Thus, a still imagecan be displayed in the display portion even with a small number ofwriting of image signals. The frequency of writing of signals can bereduced, which leads to a reduction in power consumption. Note that thepixel configuration is not limited to that illustrated in FIG. 15. Forexample, the pixel illustrated in FIG. 15 can further include a switch,a resistor, a capacitor, a transistor, a logic circuit, or the like.

Next, a cross-sectional structure of the pixel will be described withreference to FIGS. 16A to 16C. Note that a thin film transistorincluding a highly-purified oxide semiconductor layer may be used as adriving TFTs 7011, 7021, and 7001 illustrated in FIGS. 16A, 16B, and16C, respectively. For example, any of the thin film transistorsdescribed in Embodiments 2 to 10 can be used.

Light-emitting elements illustrated in this embodiment each have astructure in which an EL layer is sandwiched between a first electrodeand a second electrode.

As for the first electrode and second electrode of the light-emittingelement, as an electrode serving as a cathode, the following materialhaving a low work function is preferable, specific examples of which area rare earth metal such as Yb or Er as well as an alkali metal such asLi or Cs, an alkaline earth metal such as Mg, Ca, or Sr, and an alloycontaining any of the above metals (e.g., Mg:Ag or Al:Li); as anelectrode serving as an anode, a material having a high work function ispreferable, examples of which are titanium nitride, ZrN, Ti, W, Ni, Pt,Cr, and transparent conductive materials such as ITO, IZO (indium zincoxide), and ZnO. Note that in the case where an electron injection layeris formed in contact with the anode or the case where a hole injectionlayer is formed in contact with the anode, an influence of the workfunction of the electrode material can be reduced. The electroninjection layer and the hole injection layer may be formed using, forexample, a composite material of an organic compound and metal oxide,metal oxide, or a composite material of an organic compound and analkali metal, an alkaline earth metal, or a compound thereofAlternatively, the electron injection layer and the hole injection layercan be formed by combining those materials as appropriate.

The EL layer formed over the first electrode may include a single layeror a plurality of layers. In the case where the EL layer includes aplurality of layers, a structure in which an anode, a hole injectionlayer, a hole transport layer, a light-emitting layer, an electrontransport layer, an electron injection layer, and a cathode are stackedin this order and in contact with one another can be given as anexample. Note that not all of these layers are necessarily provided.Alternatively, a structure may be employed in which a plurality of ELlayers which are separated by an intermediate layer serving as a chargegeneration layer are provided between the first electrode and the secondelectrode.

At least one of the first electrode and the second electrode is formedusing a conductive film having light-transmitting properties so thatlight emission is extracted from the light-emitting element. Accordingto the classification based on the direction in which light emitted fromthe light-emitting element formed over a substrate is extracted, thereare three typical structures of light-emitting elements; a top emissionstructure in which light is extracted from the side where thelight-emitting element is formed, a bottom emission structure in whichlight is extracted from the substrate side, and a dual emissionstructure in which light is emitted from both the substrate side and theside where the light-emitting element is formed.

In the case where the EL layer is stacked over the first electrode, aperipheral portion of the first electrode is covered with a partition.The partition is formed using, for example, an organic resin film ofpolyimide, acrylic, polyamide, or epoxy; an inorganic insulating film,or organic polysiloxane. The partition is preferably formed using aphotosensitive material for the following reason: if an opening isformed in the photosensitive resin material which covers the firstelectrode with the partition left in the peripheral portion of the firstelectrode, a side surface from the partition to the opening is formed asan inclined surface with continuous curvature and a step of forming aresist mask can be omitted.

Note that a color filter can be formed between the substrate and thelight-emitting element. The color filter may be formed by a dropletdischarge method such as an inkjet method, a printing method, an etchingmethod using a photolithography technique, or the like.

Further, it is preferable that an overcoat layer be formed over thecolor filter and a protective insulating layer be additionally formed.The provision of the overcoat layer allows unevenness caused by thecolor filter to be eliminated. The provision of the protectiveinsulating film can prevent impurities from being dispersed from thecolor filter to the light-emitting element.

Note that in the case where the light-emitting element is formed overthe protective insulating layer, the overcoat layer, and the insulatinglayer which are formed over the thin film transistor, a contact holewhich penetrates the protective insulating layer, the overcoat layer,and the insulating layer and reaches a source electrode layer or a drainelectrode layer of the thin film transistor is formed. In particular,the contact hole is preferably formed at a position overlapping with theabove-described partition, in which case aperture ratio can be preventedfrom being reduced.

A structure of the pixel including a light-emitting element having abottom emission structure will be described. FIG. 16A is across-sectional view of the driving TFT 7011 and a light-emittingelement 7012 which are provided in the pixel.

In the driving TFT 7011, an insulating layer, an oxide semiconductorlayer, a source and drain electrode layers, a gate insulating layer, anda gate electrode layer are provided over a substrate, and a wiring layeris provided so as to be electrically connected to each of the source anddrain electrode layers.

An insulating layer 7031 is formed covering the driving TFT 7011, and acolor filter 7033 having opening is provided over the insulating layer7031. A light-transmitting conductive film 7017 is formed over anovercoat layer 7034 and an insulating layer 7035 which are formedcovering the color filter 7033. Note that the drain electrode of thedriving TFT 7011 and the conductive film 7017 are electrically connectedto each other through an opening formed in the overcoat layer 7034, theinsulating layer 7035, and the insulating layer 7031. Note that a firstelectrode 7013 of the light-emitting element 7012 is provided on and incontact with the conductive film 7017.

In the light-emitting element 7012, an EL layer 7014 is sandwichedbetween the first electrode 7013 and the second electrode 7015.

As the light-transmitting conductive film 7017, a light-transmittingconductive film such as a film of indium oxide containing tungstenoxide, indium zinc oxide containing tungsten oxide, indium oxidecontaining titanium oxide, indium tin oxide containing titanium oxide,indium tin oxide (hereinafter referred to as ITO), indium zinc oxide, orindium tin oxide to which silicon oxide is added can be used.

Here, the case where the first electrode 7013 of the light-emittingelement 7012 is used as a cathode will be described. In the case wherethe first electrode 7013 is used as a cathode, it is preferably formedusing a metal having a low work function. In FIG. 16A, the firstelectrode 7013 is formed to have a thickness that allows lighttransmission (preferably, approximately 5 nm to 30 nm). For example, analuminum film or an Mg—Ag alloy film with a thickness of 20 nm is usedfor the first electrode 7013.

Note that the light-transmitting conductive film 7017 and the firstelectrode 7013 are preferably formed in such a manner that alight-transmitting conductive film and an aluminum film are stacked andselectively etched, in which case the light-transmitting conductive film7017 and the first electrode 7013 can be etched using the same mask.

For the second electrode 7015 formed over the EL layer 7014, a materialhaving a high work function is preferably used. Further, alight-blocking film 7016, for example, a metal which blocks light, ametal which reflects light, or the like is provided over the secondelectrode 7015. In this embodiment, an ITO film is used as the secondelectrode 7015 and a Ti film is used as the light-blocking film 7016.

The color filter 7033 is covered with the overcoat layer 7034, and isadditionally covered with the protective insulating layer 7035. Althoughthe overcoat layer 7034 with a small thickness is illustrated in FIG.16A, the overcoat layer 7034 has a function to eliminate unevennesscaused by the color filter 7033.

Further, the contact hole which is formed in the overcoat layer 7034 andthe protective insulating layer 7035 and reaches a drain electrode layer7030 is located at a position overlapping with a partition 7019.

In the case of the pixel structure illustrated in FIG. 16A, light isemitted from the light-emitting element 7012 to the first electrode 7013side to pass through the color filter 7033 to the outside of the displaydevice as indicated by arrows.

Note that FIG. 16A illustrates an example in which the driving TFT 7011includes the gate electrode layer, the source electrode layer, and alight-transmitting conductive film as the drain electrode layer. Thus,part of light emitted from the light-emitting element 7012 passesthrough the color filter 7033 and the driving TFT 7011 to be extracted.

Next, a structure of the pixel including a light-emitting element havinga dual emission structure will be described. FIG. 16B is across-sectional view of the driving TFT 7021 and a light-emittingelement 7022 which are provided in the pixel.

In the driving TFT 7021, an insulating layer, an oxide semiconductorlayer, a source and drain electrode layers, a gate insulating layer, anda gate electrode layer are provided over a substrate, and a wiring layeris provided so as to be electrically connected to each of the source anddrain electrode layers.

An insulating layer 7041 is formed covering the driving TFT 7021, and acolor filter 7043 having opening is provided over the insulating layer7041. A light-transmitting conductive film 7027 is formed over anovercoat layer 7044 and an insulating layer 7045 which are formedcovering the color filter 7043. Note that the drain electrode of thedriving TFT 7021 and the conductive film 7027 are electrically connectedto each other through an opening formed in the overcoat layer 7044, theinsulating layer 7045, and the insulating layer 7041. Note that a firstelectrode 7023 of the light-emitting element 7022 is provided on and incontact with the conductive film 7027.

In the light-emitting element 7022, an EL layer 7024 is sandwichedbetween the first electrode 7023 and the second electrode 7025.

Here, the case where the first electrode 7023 of the light-emittingelement 7022 is used as a cathode will be described. Thelight-transmitting conductive film 7027 may be formed in a mannersimilar to that of the conductive film illustrated in FIG. 16A. Thefirst electrode 7023 may be formed in a manner similar to that of thefirst electrode 7013 illustrated in FIG. 16A. The EL layer 7024 may beformed in a manner similar to that of the EL layer 7014 illustrated inFIG. 16A. Therefore, detailed description of those layers is not givenhere.

The second electrode 7025 formed over the EL layer 7024 functions as ananode here, it is preferably formed using a material having a high workfunction, such as a transparent conductive material (e.g., ITO, IZO, orZnO). In this embodiment, ITO is formed as the second electrode 7025.

A color filter 7043, an overcoat layer 7044, and a protective insulatinglayer 7045 may be formed in manners similar to those of the color filter7033, the overcoat layer 7034, and the protective insulating layer 7035included in the pixel illustrated in FIG. 16A.

In the case of the pixel structure illustrated in FIG. 16B, light isemitted from the light-emitting element 7022 to both the first electrode7023 side and the second electrode 7025 side as indicated by arrows, andthe light on the first electrode 7023 side passes through the colorfilter 7043 to the outside of the display device.

Note that FIG. 16B illustrates an example in which the driving TFT 7021includes the gate electrode layer, the source electrode layer, and alight-transmitting conductive film as the drain electrode layer. Thus,part of light emitted from the light-emitting element 7022 passesthrough the color filter 7043 and the driving TFT 7021 to be extracted.

Further, a contact hole which is formed in the overcoat layer 7044 andthe insulating layer 7045 and reaches a drain electrode layer 7040 islocated at a position overlapping with a partition 7029. The contacthole which reaches the drain electrode layer and the partition 7029overlap with each other, whereby the aperture ratio on the secondelectrode 7025 side can be substantially the same as that on the firstelectrode 7023 side.

Note that in the case where full color display is performed on bothdisplay surfaces of the light-emitting element having a dual emissionstructure, light from the second electrode 7025 side does not passthrough the color filter layer 7043; therefore, a sealing substrateprovided with another color filter layer is preferably provided on thesecond electrode 7025.

Next, a structure of the pixel including a light-emitting element havinga top emission structure will be described. FIG. 16C is across-sectional view of the driving TFT 7001 and a light-emittingelement 7002 which are provided in the pixel.

In the driving TFT 7001, an insulating layer, an oxide semiconductorlayer, a source and drain electrode layers, a gate insulating layer, anda gate electrode layer are provided over a substrate, and a wiring layeris provided so as to be electrically connected to each of the source anddrain electrode layers.

An insulating layer 7051 is formed covering the driving TFT 7001, and aninsulating layer 7053 having an opening is provided over the insulatinglayer 7051. A first electrode 7003 is formed over an insulating layer7055 which is formed covering the insulating layer 7053. Note that thedrain electrode of the driving TFT 7001 and the first electrode 7003 areelectrically connected to each other through an opening formed in theinsulating layer 7055 and an insulating layer 7051.

Note that the insulating layer 7053 can be formed using a resin materialsuch as polyimide, acrylic, benzocyclobutene, polyamide, or epoxy. Inaddition to the resin materials given above, it is also possible to usea low-dielectric constant material (a low-k material), a siloxane-basedresin, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), orthe like. Note that the insulating layer 7053 may be formed by stackinga plurality of insulating films formed of these materials. There is noparticular limitation on the method for forming the insulating layer7053. The planarization insulating layer can be formed, depending on thematerial, by a method such as a sputtering method, an SOG method, spincoating, dip coating, spray coating, or a droplet discharge method (suchas an inkjet method, screen printing, offset printing, or the like), ora tool (equipment) such as a doctor knife, a roll coater, a curtaincoater, or a knife coater. For example, the formation of the insulatinglayer 7053 allows unevenness caused by the driving TFT to be eliminated.

In the light-emitting element 7002, an EL layer 7004 is sandwichedbetween the first electrode 7003 and the second electrode 7005. As forthe light-emitting element 7002 illustrated in FIG. 16C, the case wherethe first electrode 7003 is used as a cathode will be described.

A material similar to that of the first electrode 7013 illustrated inFIG. 16A may be used for the first electrode 7003. However, it ispreferable that in the light-emitting element having a top emissionstructure illustrated in FIG. 16C, the first electrode 7003 do not havelight-transmitting properties but be an electrode having highreflectance. The use of an electrode having high reflectance makes itpossible to increase light extraction efficiency.

As the first electrode 7003, for example, an aluminum film, an alloyfilm containing aluminum as a main component, or a layer in which atitanium film is stacked over an aluminum film is preferably used. InFIG. 16C, a layer in which a Ti film, an aluminum film, and a Ti filmare stacked in this order is used as the first electrode 7003.

The EL layer 7004 may be formed in a manner similar to that of the ELlayer 7014 illustrated in FIG. 16A. The second electrode 7005 may beformed in a manner similar to that of the second electrode 7025illustrated in FIG. 16B. Therefore, detailed description of those layersis not given here.

In the case of the pixel structure illustrated in FIG. 16C, light isemitted from the light-emitting element 7002 to the second electrode7005 side as indicated by arrows.

In the case where full color display is performed using the structureillustrated in FIG. 16C, for example, the light-emitting element 7002 isused as a green light-emitting element, one of adjacent light-emittingelements is used as a red light-emitting element, and the other of theadjacent light-emitting element is used as a blue light-emittingelement. Alternatively, a light-emitting display device capable of fullcolor display may be manufactured using four kinds of light-emittingelements which include a white light-emitting element in addition tothree kinds of light-emitting elements.

Alternatively, a light-emitting display device capable of full colordisplay may be manufactured in such a way that all of a plurality oflight-emitting elements which are arranged in the structure illustratedin FIG. 16C is white light-emitting elements and a sealing substratehaving a color filter or the like is arranged on the light-emittingelements including the light-emitting element 7002. A element whichexhibits a monochromatic color such as white is formed and combined witha color filter or a color conversion layer, whereby full color displaycan be performed.

Needless to say, display of monochromatic light may also be performed.For example, a lighting device may be formed with the use of white lightemission, or an area-color light-emitting device may be formed with theuse of a monochromatic color light emission.

If necessary, an optical film such as a polarizing film including acircularly polarizing plate may be provided.

Note that, although the organic EL elements are described here as thelight-emitting elements, an inorganic EL element can also be provided asa light-emitting element.

Note that the example is described in which the thin film transistor(the driving TFT) which controls the driving of the light-emittingelement is electrically connected to the light-emitting element;however, a structure may be employed in which a TFT for current controlis connected between the driving TFT and the light-emitting element.

Next, the appearance and cross section of a light-emitting display panel(also referred to as a light-emitting panel) which corresponds to oneembodiment of the display device will be described with reference toFIGS. 17A and 17B. FIG. 17A is a top view of a panel in which a thinfilm transistor and a light-emitting element formed over a firstsubstrate are sealed between the first substrate and a second substratewith a sealant. FIG. 17B is a cross-sectional view taken along line H-Iof FIG. 17A.

A sealant 4505 is provided so as to surround a pixel portion 4502,signal line driver circuits 4503 a and 4503 b, and scan line drivercircuits 4504 a and 4504 b which are provided over a first substrate4501. In addition, a second substrate 4506 is provided over the pixelportion 4502, the signal line driver circuits 4503 a and 4503 b, and thescan line driver circuits 4504 a and 4504 b. Accordingly, the pixelportion 4502, the signal line driver circuits 4503 a and 4503 b, and thescan line driver circuits 4504 a and 4504 b are sealed together with afiller 4507, by the first substrate 4501, the sealant 4505, and thesecond substrate 4506. It is preferable that a panel be packaged(sealed) with a protective film (such as a laminate film or anultraviolet curable resin film) or a cover material with highair-tightness and little degasification so that the panel is not exposedto the outside air, in this manner.

The pixel portion 4502, the signal line driver circuits 4503 a and 4503b, and the scan line driver circuits 4504 a and 4504 b formed over thefirst substrate 4501 each include a plurality of thin film transistors,and a thin film transistor 4510 included in the pixel portion 4502 and athin film transistor 4509 included in the signal line driver circuit4503 a are illustrated as an example in FIG. 17B. An insulating layer4542 is provided over the thin film transistors 4509 and 4510. Inaddition, a source electrode layer or a drain electrode layer of thethin film transistor 4510 is electrically connected to a first electrodelayer 4517 of a light-emitting element 4511 through a contact holeformed in the insulating layer 4542.

Any of the thin film transistors including a highly-purified oxidesemiconductor layer which are described in Embodiments 1 to 10 is usedas the thin film transistors 4509 and 4510.

A conductive layer 4540 is provided over part of an insulating layer4542, which overlaps with a channel formation region of an oxidesemiconductor layer in the thin film transistor 4509 for a drivercircuit. The provision of the conductive layer 4540 in the positionoverlapping with the channel formation region of the oxide semiconductorlayer makes it possible to reduce the amount of change in thresholdvoltage of the thin film transistor 4509 before and after a BT stresstest (a bias-temperature stress test). In this specification, the BTstress test (the bias-temperature stress test) refers to a test in whicha high gate voltage is applied to a thin film transistor in ahigh-temperature atmosphere. Further, the potential of the conductivelayer 4540 may be the same as or different from that of a gate electrodelayer of the thin film transistor 4509. The conductive layer 4540 canfunction also as a second gate electrode layer. Alternatively, thepotential of the conductive layer 4540 may be GND or 0 V, or theconductive layer 4540 may be in a floating state.

Note that the structure of the light-emitting element 4511 is notlimited to a stacked-layer structure of the first electrode layer 4517,an electroluminescent layer 4512, and a second electrode layer 4513. Thestructure of the light-emitting element 4511 can be changed asappropriate depending on the direction in which light is extracted fromthe light-emitting element 4511, or the like.

A partition 4520 is formed using an organic resin film, an inorganicinsulating film, or organic polysiloxane. It is particularly preferablethat the partition 4520 be formed using a photosensitive material and anopening be formed over the first electrode layer 4517 so that a sidewallof the opening is formed as an inclined surface with continuouscurvature.

The electroluminescent layer 4512 may be formed as a single layer or aplurality of layers stacked.

A protective film may be formed over the second electrode layer 4513 andthe partition 4520 in order to prevent entry of oxygen, hydrogen,moisture, carbon dioxide, or the like into the light-emitting element4511. As the protective film, a silicon nitride film, a silicon nitrideoxide film, a DLC film, or the like can be formed.

Further, a variety of signals and potentials are supplied to the signalline driver circuits 4503 a and 4503 b, the scan line driver circuits4504 a and 4504 b, or the pixel portion 4502 from FPCs 4518 a and 4518b.

A connection terminal electrode 4515 is formed using the same conductivefilm as the first electrode layer 4517 included in the light-emittingelement 4511. A terminal electrode 4516 is formed using the sameconductive film as the source and drain electrode layers included in thethin film transistors 4509 and 4510.

The connection terminal electrode 4515 is electrically connected to aterminal included in the FPC 4518 a through an anisotropic conductivefilm 4519.

The substrate located in the direction in which light emitted from thelight-emitting element 4511 is extracted needs to havelight-transmitting properties. In that case, a light-transmittingmaterial such as a glass plate, a plastic plate, a polyester film, or anacrylic film is used for the second substrate 4506.

As the filler 4507, an ultraviolet curable resin or a thermosettingresin can be used, in addition to an inert gas such as nitrogen orargon. For example, PVC (polyvinyl chloride), acrylic, polyimide, anepoxy resin, a silicone resin, PVB (polyvinyl butyral), or EVA (ethylenevinyl acetate) can be used. For example, nitrogen is used for thefiller.

Further, if needed, an optical film, such as a polarizing plate, acircularly polarizing plate (including an elliptically polarizingplate), a retardation plate (a quarter-wave plate or a half-wave plate),or a color filter, may be provided as appropriate on a light-emittingsurface of the light-emitting element. The polarizing plate or thecircularly polarizing plate may be provided with an anti-reflectionfilm. For example, anti-glare treatment by which reflected light can bediffused by projections and depressions on the surface so as to reducethe glare can be performed.

The signal line driver circuits 4503 a and 4503 b and the scan linedriver circuits 4504 a and 4504 b may be mounted as driver circuitsformed using a single crystal semiconductor film or a polycrystallinesemiconductor film over a substrate separately prepared. Alternatively,only the signal line driver circuits or part thereof, or only the scanline driver circuits or part thereof may be separately formed andmounted. This embodiment is not limited to the structure illustrated inFIGS. 17A and 17B.

The above-described structure makes it possible to provide a displaydevice in which the thin film transistor in which the highly-purifiedoxide semiconductor layer is included and off-state current is reducedis used. Since the thin film transistor in which off-state current isreduced is used in a pixel, a capacitor provided in the pixel can holdvoltage for a long time. As a result, a display device whose operationat the time of display of a still image or the like is stabilized andwhich consumes less power can be provided.

This embodiment can be implemented in combination with any of thestructures of the other embodiments as appropriate.

(Embodiment 12)

In this embodiment, one embodiment of a display device in which alight-storing layer is provided in a pixel portion will be describedwith reference to FIG. 18.

FIG. 18 is a cross-sectional view of a pixel portion having a bottomemission structure and illustrates a driving TFT 7211 and alight-emitting element 7212 which are provided in a pixel.

In the driving TFT 7211, an insulating layer, an oxide semiconductorlayer, a source and drain electrode layers, a gate insulating layer, anda gate electrode layer are provided over a substrate, and a wiring layeris provided so as to be electrically connected to each of the source anddrain electrode layers.

Further, an insulating layer 7231 is formed covering the driving TFT7211, and a light-storing layer 7233 having an opening is provided overthe insulating layer 7231. A light-transmitting conductive film 7217 isformed over an overcoat layer 7234 and an insulating layer 7235 whichare formed covering the light-storing layer 7233. Note that a drainelectrode 7230 of the driving TFT 7211 and the conductive film 7217 areelectrically connected to each other through an opening formed in theovercoat layer 7234, the insulating layer 7235, and the insulating layer7231. Note that a first electrode 7213 of the light-emitting element7212 is provided on and in contact with the conductive film 7217.

The driving TFT 7211 and the light-emitting element 7212 can be formedby the method described in Embodiment 11. Therefore, the detaileddescription is not given here.

The light-storing layer 7233 contains a light-storing material andstores light emitted from the light-emitting element adjacent thereto.Even after the light-emitting element stops emitting light, thelight-storing material contained in the light-storing layer 7233continues to emit light. In this embodiment, as the light-storingmaterial, copper activated zinc sulfide (ZnS:Cu) is used. Alternatively,a phosphor in which an activator is added using sulfide such asstrontium sulfide (SrS) as a base material; alkaline earth aluminate inwhich a rare earth element is added as an activator, such as CaAl₂O₄:Eu,CaAl₂O₄:Nd, Sr₄ Al₁₄O₂₅:Eu, Sr₄Al₁₄O₂₅:Dy, SrAl₂O₄:Eu, or ArAl₂O₄:Dy; orthe like may be used.

The period in which the light-storing layer 7233 keeps emitting lightcan be changed depending on the kind of light-storing material to beused. The period in which the light-storing layer 7233 keeps emittinglight, that is, the light-remaining period depends on the kind oflight-storing material; therefore, the material is selected inaccordance with an intended purpose. For example, for a display deviceused in the case where displayed contents do not need to be changedfrequently, a light-storing material which has long light-remaining timemay be selected. In addition, in the case where display is changedrelatively frequently, a light-storing material which has shortlight-remaining time may be selected. Note that in the case where thelight-storing material is an inorganic particle, it has a grain diameterof greater than or equal to 1 nm and less than or equal to 10 μm,preferably greater than or equal to 10 nm and less than or equal to 5μm. The reason for the range of grain diameter given above is asfollows: if the grain size is less than or equal to 1 nm, light-storingproperties is lost, and if the grain size is greater than or equal to 10μm, the planarity of the light-storing layer is damaged, which makes itdifficult to manufacture a light-emitting element.

In addition, in this embodiment, the light-storing layer 7233 contains abinder polymer and is formed using a dispersion liquid in whichlight-storing materials are dispersed, by selecting a droplet dischargemethod such as an ink-jet method, a printing method, a spin coatingmethod, an etching method using a photolithography technique, or thelike as appropriate.

Further, the light-storing layer 7233 is covered with the overcoat layer7234, and the overcoat layer 7234 is covered with the insulating layer7235. Note that although the overcoat layer 7234 is illustrated to havea small thickness in FIG. 18, the overcoat layer 7234 has a function ofeliminating the unevenness of the light-storing layer 7233.

Note that the position where the light-storing layer is provided is notlimited to the position between a user of the display device and thelight-emitting element. For example, a light-emitting element having adual emission structure in which an EL layer is sandwiched between apair of electrodes having light-transmitting properties haslight-transmitting properties. In the case where the light-emittingelement has light-transmitting properties as described above, thelight-storing layer can be arranged on the back surface side of alight-emitting layer from the side of the user of the display device. Inother words, the light-emitting element can be provided between thelight-storing layer and the user of the display device. When thelight-emitting element is provided between the user of the displaydevice and the light-storing layer, the light-storing layer does notnecessarily need to have light-transmitting properties. Thus, thelight-storing material can be selected from a wide range of materials.Specifically, a light-storing material with a grain size of less than orequal to 100 μm can be used.

The above-described structure makes it possible to provide a displaydevice in which the light-storing layer and the thin film transistor inwhich a highly-purified oxide semiconductor layer is included are usedin the pixel portion. Since the thin film transistor in which off-statecurrent is reduced is used in a pixel of the display device, a capacitorprovided in the pixel can hold voltage for a long time. As a result, adisplay device whose operation at the time of display of a still imageor the like is stabilized and which consumes low power can be provided.

Further, according to one embodiment of the present invention, aself-luminous display device that is excellent in visibility and candisplay images even in an environment in which outside light is weak canbe provided. In addition, a display device in which flicker is notnoticeable even when an emission interval of the light-emitting elementis long can be provided because the light-storing layer in which alight-storing material is used is included in the pixel portion.Moreover, a display device which can be kept using for a long time canbe provided because energy can be given to a light-storing material bydriving a light-emitting element even in an environment in which outsidelight is weak.

This embodiment can be implemented in combination with any of thestructures of the other embodiments as appropriate.

(Embodiment 13)

In this embodiment, examples of electronic devices each including thedisplay device described in the above embodiment will be described.

FIG. 19A illustrates a portable game machine, which includes a chassis9630, a display portion 9631, a speaker 9633, operation keys 9635, aconnection terminal 9636, a recording medium reading portion 9672, andthe like. The portable game machine illustrated in FIG. 19A can have afunction of reading a program or data stored in a recording medium todisplay on the display portion; a function of sharing information bywireless communication with another portable game machine; and the like.Note that the portable game machine illustrated in FIG. 19A can have avariety of functions without limitation to the above-describedfunctions.

FIG. 19B illustrates a digital camera, which includes a chassis 9630, adisplay portion 9631, a speaker 9633, operation keys 9635, a connectionterminal 9636, a shutter button 9676, an image receiving portion 9677,and the like. The digital camera having the television receivingfunction, which is illustrated in FIG. 19B, can have a function ofphotographing a still image; a function of shooting a moving image; afunction of automatically or manually correcting the photographed image;a function of obtaining a variety of information from an antenna; afunction of holding the photographed image or the information obtainedfrom the antenna; a function of displaying the photographed image or theinformation obtained from the antenna on the display portion; and thelike. Note that the digital camera having the television receivingfunction illustrated in FIG. 19B can have a variety of functions withoutlimitation to the above-described functions.

FIG. 19C illustrates a television receiver, which includes a chassis9630, a display portion 9631, a speaker 9633, operation keys 9635, aconnection terminal 9636, and the like. The television receiverillustrated in FIG. 19C can have a function of processing an electricwave for television and converting the electric wave into a pixelsignal; a function of processing the pixel signal and converting thepixel signal into a signal suitable for display; a function ofconverting a frame frequency of the pixel signal; and the like. Notethat the television receiver illustrated in FIG. 19C can have a varietyof functions without limitation the to the above-described functions.

FIG. 20A illustrates a computer, which includes a chassis 9630, adisplay portion 9631, a speaker 9633, operation keys 9635, a connectionterminal 9636, a pointing device 9681, an external connection port 9680,and the like. The computer illustrated in FIG. 20A can have a functionof displaying various kinds of information (e.g., a still image, amoving image, and a text image) on the display portion; a function ofcontrolling processing by various kinds of software (programs); acommunication function such as wireless communication or wirecommunication; a function of connecting with various computer networksby using the communication function; a function of transmitting orreceiving various kinds of data by using the communication function; andthe like. Note that the computer illustrated in FIG. 20A can have avariety of functions without limitation to the above-describedfunctions.

FIG. 20B illustrates a mobile phone, which includes a chassis 9630, adisplay portion 9631, a speaker 9633, operation keys 9635, a microphone9638, and the like. The mobile phone illustrated in FIG. 20B can have afunction of displaying various kinds of information (e.g, a still image,a moving image, and a text image); a function of displaying a calendar,the date, the time, and the like on the display portion; a function ofoperating or editing the information displayed on the display portion; afunction of controlling processing by various kinds of software(programs); and the like. Note that the mobile phone illustrated in FIG.20B can have a variety of functions without limitation to theabove-described functions.

FIG. 20C illustrates an electric device including electronic paper (theelectric device is also referred to as an e-book reader), which includesa chassis 9630, a display portion 9631, operation keys 9635, and thelike. The electronic paper illustrated in FIG. 20C can have variousfunctions such as a function of displaying various kinds of information(e.g, a still image, a moving image, and a text image); a function ofdisplaying a calendar, the date, the time, and the like on the displayportion; a function of operating or editing the information displayingon the display portion; and a function of controlling processing byvarious kinds of software (programs). Note that the electronic paperillustrated in FIG. 20C can have a variety of functions withoutlimitation to the above-described functions.

As for the electronic devices described in this embodiment, off-statecurrent can be reduced in a plurality of pixels included in the displayportion. Thus, voltage can be held in a capacitor for a long time, whichresults in an electronic device provided with a display device whichconsumes less power at the time of displaying a still image or the like.In addition, an increase in aperture ratio is achieved, which results ina display device including a high-definition display portion.

Further, according to one embodiment of the present invention, aself-luminous display device that is excellent in visibility and candisplay images even in an environment in which outside light is weak canbe provided. In addition, in the case where the light-storing layer inwhich a light-storing material is used is included in the pixel portion,a display device in which flicker is not noticeable even when anemission interval of the light-emitting element is long can be provided.Moreover, a display device which can be kept using for a long time canbe provided because energy can be given to a light-storing material bydriving a light-emitting element even in an environment in which outsidelight is weak.

This embodiment can be implemented in combination with any of thestructures of the other embodiments as appropriate.

(Embodiment 14)

In this embodiment, a block diagram of a display device, and shutdownprocedure and startup procedure of operation in a driver circuit will bedescribed. First, a block diagram of the display device will bedescribed with reference to FIG. 25.

A display device 1000 described in this embodiment includes a displaypanel 1001, a signal generation circuit 1002, a memory circuit 1003, acomparison circuit 1004, a selection circuit 1005, and a display controlcircuit 1006.

The display panel 1001 includes, for example, a driver circuit portion1007 and a pixel portion 1008. A gate line driver circuit 1009A and asignal line driver circuit 1009B are included. The gate line drivercircuit 1009A and the signal line driver circuit 1009B are drivercircuits for driving the pixel portion 1008 including a plurality ofpixels. The gate line driver circuit 1009A, the signal line drivercircuit 1009B, and the pixel portion 1008 may be formed using thin filmtransistors formed over one substrate.

Note that an n-channel thin film transistor in which a semiconductorlayer is formed of an oxide semiconductor is used as some or all of thinfilm transistors included in the gate line driver circuit 1009A, thesignal line driver circuit 1009B, and the pixel portion 1008. Note thatthe gate line driver circuit 1009A or the signal line driver circuit1009B in the driver circuit portion 1007 may be formed over the samesubstrate or different substrates.

The signal generation circuit 1002 is a circuit which generates a pulsesignal for outputting a signal for performing a display in the pixelportion 1008 by the gate line driver circuit 1009A and the signal linedriver circuit 1009B. In addition, the signal generation circuit 1002 isa circuit for outputting a signal to the driver circuit portion 1007through a wiring and a circuit for outputting an image signal (alsoreferred to as video voltage, a video signal, and video data) to thememory circuit 1003 through a wiring. In other words, the signalgeneration circuit 1002 is a circuit for generating and outputting acontrol signal for driving the driver circuit portion 1007 and an imagesignal to be supplied to the pixel portion.

Specifically, the signal generation circuit 1002 supplies, as a controlsignal, a high power supply potential Vdd that is power supply voltageto the gate line driver circuit 1009A and the signal line driver circuit1009B. In addition, the signal generation circuit 1002 generates startpulse SP and a clock signal CK for the gate line driver circuit 1009A tooutput them to the gate line driver circuit 1009A, and generates startpulse SP and a clock signal CK for the signal line driver circuit 1009Bto output them to the signal line driver circuit 1009B. Moreover, thesignal generation circuit 1002 outputs image signal data for displayinga moving image or a still image to the memory circuit 1003.

Note that a moving image refers to an image which is recognized as amoving image by the human eye by switching a plurality of images at highspeed which are time-divided into a plurality of frames. Specifically, amoving image refers to a series of image signals which can be recognizedas a moving image with less flicker by the human eye by switching imagesat least 60 times (60 frames) per second. In contrast, a still imagerefers to image signals which do not change in a series of frameperiods, for example, in the n-th frame and (n+1)th frame, unlike themoving image, a plurality of images which are time-divided into aplurality of frames are switched at high speed.

Note that the signal generation circuit 1002 may generate other signalssuch as an image signal and a latch signal. The signal generationcircuit 1002 may output, to the gate line driver circuit 1009A and/orthe signal line driver circuit 1009B, a reset signal Res for stoppingoutput of a pulse signal to each driver circuit. Note that each signalmay be composed of a plurality of signals such as a first clock signaland a second clock signal.

Note that high power supply potential Vdd refers to a potential which ishigher than a reference potential, and a low power supply potentialrefers to a potential which is lower than or equal to the referencepotential. Note that both the high power supply potential and the lowpower supply potential are preferably potential such that a transistorcan operate.

Note that voltage refers to a potential difference between a givenpotential and a reference potential (e.g., a ground potential) in manycases. Accordingly, voltage, potential and a potential difference can bereferred to as potential, voltage, and a voltage difference,respectively.

Note that in the case where an image signal output from the signalgeneration circuit 1002 to the memory circuit 1003 is an analog signal,the signal may be switched into a digital signal through an A/Dconverter or the like to be output to the memory circuit 1003.

The memory circuit 1003 includes a plurality of frame memory 1010 forstoring image signals for a plurality of frames. The number of framememory included in the memory circuit 1003 is not particularly limitedand may be an element that can store the image signals on a plurality offrames. Note that the frame memory may be formed using a memory elementsuch as dynamic random access memory (DRAM) or static random accessmemory (SRAM).

Note that the number of frame memories 1010 is not particularly limitedas long as an image signal can be stored for each frame period. Inaddition, the image signals of the frame memories 1010 are selectivelyread out by the comparison circuit 1004 and the selection circuit 1005.

The comparison circuit 1004 is a circuit which selectively reads outimage signals in a series of frame periods stored in the memory circuit1003, compares the image signals, and detects a difference thereof Inthe case where the difference is detected by the comparison of the imagesignals by the comparison circuit 1004, a moving image is recognized inthe series of frame periods from which the different is detected,whereas, in the case where the difference is not detected by thecomparison of the image signals by the comparison circuit 1004, a stillimage is recognized in the series of frame periods from which thedifference is not detected. In other words, the detection of thedifferent by the comparison circuit 1004 determines whether the imagesignals in the series of frame periods are image signals for displayinga moving image or image signals for displaying a still image. Note thatthe difference obtained by the comparison may be set to be detected whenthe difference exceeds a certain level.

The selection circuit 1005 includes a plurality of switches such as thinfilm transistors, and is a circuit which selects, when image signals fordisplaying a moving image are determined by the difference detection inthe comparison circuit 1004, the image signals from the frame memories1010 in which the image signals are stored, and outputs to the displaycontrol circuit 1006. Note that in the case were a difference of imagesignals between frames compared by the comparison circuit is notdetected, an image displayed in the frames is a still image. In thatcase, a structure may be employed in which an image signal of the frameperiod is not output to the display control circuit 1006.

The display control circuit 1006 is a circuit which switches supplyingand stop of supplying of the image signal and the control signal such asthe high power supply potential Vdd, the low power supply potential Vss,the start pulse SP, the clock pulse CK, and the reset signal Res to thedriver circuit portion 1007. Specifically, when an image is determinedto be a moving image by the comparison circuit 1004, that is, adifference of image signals in a series of frames is detected, the imagesignals are supplied from the selection circuit 1005 to the drivercircuit portion 1007 through the display control circuit 1006, and thecontrol signals are supplied to the driver circuit portion 1007 throughthe display control circuit 1006. On the other hand, when an image isdetermined to be a still image by the comparison circuit 1004, that is,a difference of image signals in a series of frames is not detected, theimage signal of the latter frame is not supplied from the selectioncircuit 1005, so that the image signal is not supplied to the drivercircuit portion 1007 through the display control circuit 1006, and thedisplay control circuit 1006 stops supplying the control signals to thedriver circuit portion 1007.

Note that in the case where the still image is determined, when theperiod during which an image is assumed to be a still image is short,stop of supplying of the high power supply potential Vdd and the lowpower supply potential Vss among the control signals is not necessarilyperformed. Stop and restart of the high power supply potential Vdd andthe low power supply potential Vss are preferably performed frequently,in which case an increase in power consumption can be reduced.

It is preferable that the stop of supplying of the image signals and thecontrol signals is performed entirely in the period for holding an imagesignal in each pixel in the pixel portion 1008, and the image signalsand the control signals which the display control circuit 1005 suppliesbefore are supplied again, such that the image signal is supplied againafter the holding period of each pixel.

The supplying of any signal refers to supplying a predeterminedpotential to a wiring. The stop of supplying of any signal refers tostop of supplying of the predetermined potential to the wiring, andconnection to a wiring to which a predetermined fixed potential issupplied, for example, a wiring to which the low power supply potentialVss is supplied. The stop of supplying of any signal also refers to cutof an electrical connection to a wiring to which a predeterminedpotential is supplied, to make a floating state.

As described above, in the thin film transistor including the oxidesemiconductor layer, off-state current per a channel width of 1 μm canbe reduced to less than or equal to 1 aA (1×10⁻¹⁸ A/μm) (1aA/μm),whereby the holding period can be extended. Thus, a synergistic effectis expected to be generated in reduction of power consumption when astill image is displayed in this embodiment.

In this manner, image signals are compared to determine whether an imagethereof is a moving image or a still image, and supplying or stop ofsupplying of control signals such as a clock signal or a start pulse isselectively performed, whereby power consumption can be reduced.

Next, an example of a structure of a shift register included in each ofthe gate line driver circuit 1009A and the signal line driver circuit1009B of the driver circuit portion 1007 is described using FIGS. 26A to26C.

The shift register shown in FIG. 26A includes first to N-th pulse outputcircuits 10_1 to 10_N (N is a natural number greater than or equal to3). A first clock signal CK1 from a first wiring 11, a second clocksignal CK2 from a second wiring 12, a third clock signal CK3 from athird wiring 13, and a fourth clock signal CK4 from a fourth wiring 14are supplied to the first to the N-th pulse output circuits 10_1 to 10_Nof the shift register shown in FIG. 26A. A start pulse SP1 (a firststart pulse) from a fifth wiring 15 is input to the first pulse outputcircuit 10_1. A signal from the pulse output circuit in the previousstage (the signal is referred to as a previous-stage signal OUT(n−1)) (nis a natural number greater than or equal to 2 and less than or equal toN) is input to the n-th pulse output circuit 10_N in the second or laterstage. A signal from the third pulse output circuit 10_3 in the stagethat is two stages after the first pulse output circuit 10_1 is input tothe first pulse output circuit 10_1. In a similar manner, a signal fromthe (n+2)-th pulse output circuit 10_(n+2) in the stage two stages afterthe n-th pulse output circuit 10_n (the signal is referred to as alater-stage signal OUT(n+2)) is input to the n-th pulse output circuit.Thus, the pulse output circuits in the respective stages output firstoutput signals (OUT(1)(SR) to OUT (N)(SR)) to be input to the pulseoutput circuits in the respective subsequent stages and/or the pulseoutput circuits in the stages that are two stages before the respectivepulse output circuits and second output signals (OUT(1) to OUT (N)) tobe input to other circuits or the like. Note that as shown in FIG. 26A,the later-stage signal OUT(n+2) is not input to the pulse outputcircuits in the last two stages of the shift register; therefore, as anexample, a second start pulse SP2 from a sixth wiring 17 may be input tothe pulse output circuit in one of the last two stages of the shiftregister and a third start pulse SP3 from a seventh wiring 18 may beinput to the pulse output circuit in the other of the last two stages ofthe shift register. Alternatively, signals may be generated inside theshift register. For example, an (N+1)-th pulse output circuit 10_(N+1)and an (N+2)-th pulse output circuit 10_(N+2) which do not contribute topulse output to the pixel portion (such circuits are also referred to asdummy stages) may be provided, and signals corresponding to the secondstart pulse (SP2) and the third start pulse (SP3) may be generated inthe dummy stages.

Note that the first to the fourth clock signals (CK1) to (CK4) each area signal which oscillates between an H-level signal and an L-levelsignal at a constant cycle. The first to the fourth clock signals (CK1)to (CK4) are delayed by ¼ period sequentially. In this embodiment, byusing the first to fourth clock signals (CK1) to (CK4), control ofdriving of the pulse output circuit or the like is performed. Note thatthe clock signal is also called GCK or SCK depending on a driver circuitto which the clock signal is input; however, description is made in thisembodiment by using CK as the clock signal.

Note that when it is explicitly described that “A and B are connected,”the case where A and B are electrically connected, the case where A andB are functionally connected, and the case where A and B are directlyconnected are included therein. Here, each of A and B corresponds to anobject (e.g., a device, an element, a circuit, a wiring, an electrode, aterminal, a conductive film, or a layer). Therefore, another connectionrelation shown in drawings and texts is included without being limitedto a predetermined connection relation, for example, the connectionrelation shown in the drawings and the texts.

Each of the first to N-th pulse output circuits 10_1 to 10_N includes afirst input terminal 21, a second input terminal 22, a third inputterminal 23, a fourth input terminal 24, a fifth input terminal 25, afirst output terminal 26, and a second output terminal 27 (see FIG.26B).

The first input terminal 21, the second input terminal 22, and the thirdinput terminal 23 are electrically connected to any of the first tofourth wirings 11 to 14. For example, in FIGS. 26A and 26B, the firstinput terminal 21 of the first pulse output circuit 10_1 is electricallyconnected to the first wiring 11, the second input terminal 22 of thefirst pulse output circuit 10_1 is electrically connected to the secondwiring 12, and the third input terminal 23 of the first pulse outputcircuit 10_1 is electrically connected to the third wiring 13. Inaddition, the first input terminal 21 of the second pulse output circuit10_2 is electrically connected to the second wiring 12, the second inputterminal 22 of the second pulse output circuit 10_2 is electricallyconnected to the third wiring 13, and the third input terminal 23 of thesecond pulse output circuit 10_2 is electrically connected to the fourthwiring 14.

In FIGS. 26A and 26B, in the first pulse output circuit 10_1, the firststart pulse SP1 is input to the fourth input terminal 24, the firstoutput signal OUT(1)(SR) is output from the first output terminal 26,and the second output signal OUT(1) is output from the second outputterminal 27.

Next, an example of a specific circuit structure of the pulse outputcircuit is described with reference to FIG. 26C.

In FIG. 26C, a first terminal of the first transistor 31 is electricallyconnected to the power supply line 51, a second terminal of the firsttransistor 31 is electrically connected to a first terminal of the ninthtransistor 39, and a gate electrode of the first transistor 31 iselectrically connected to the fourth input terminal 24. A first terminalof the second transistor 32 is electrically connected to the powersupply line 52, a second terminal of the second transistor 32 iselectrically connected to the first terminal of the ninth transistor 39,and a gate electrode of the second transistor 32 is electricallyconnected to a gate electrode of the fourth transistor 34. A firstterminal of the third transistor 33 is electrically connected to thefirst input terminal 21, and a second terminal of the third transistor33 is electrically connected to the first output terminal 26. A firstterminal of the fourth transistor 34 is electrically connected to thepower supply line 52, and a second terminal of the fourth transistor 34is electrically connected to the first output terminal 26. A firstterminal of the fifth transistor 35 is electrically connected to thepower supply line 52, a second terminal of the fifth transistor 35 iselectrically connected to the gate electrode of the second transistor 32and the gate electrode of the fourth transistor 34, and a gate electrodeof the fifth transistor 35 is electrically connected to the fourth inputterminal 24. A first terminal of the sixth transistor 36 is electricallyconnected to the power supply line 51, a second terminal of the sixthtransistor 36 is electrically connected to the gate electrode of thesecond transistor 32 and the gate electrode of the fourth transistor 34,and a gate electrode of the sixth transistor 36 is electricallyconnected to the fifth input terminal 25. A first terminal of theseventh transistor 37 is electrically connected to the power supply line51, a second terminal of the seventh transistor 37 is electricallyconnected to a second terminal of the eighth transistor 38, and a gateelectrode of the seventh transistor 37 is electrically connected to thethird input terminal 23. A first terminal of the eighth transistor 38 iselectrically connected to the gate electrode of the second transistor 32and the gate electrode of the fourth transistor 34, and a gate electrodeof the eighth transistor 38 is electrically connected to the secondinput terminal 22. The first terminal of the ninth transistor 39 iselectrically connected to the second terminal of the first transistor 31and the second terminal of the second transistor 32, a second terminalof the ninth transistor 39 is electrically connected to the gateelectrode of the third transistor 33 and the gate electrode of the tenthtransistor 40, and a gate electrode of the ninth transistor 39 iselectrically connected to the power supply line 51. A first terminal ofthe tenth transistor 40 is electrically connected to the first inputterminal 21, a second terminal of the tenth transistor 40 iselectrically connected to the second output terminal 27, and the gateelectrode of the tenth transistor 40 is electrically connected to thesecond terminal of the ninth transistor 39. A first terminal of theeleventh transistor 41 is electrically connected to the power supplyline 52, a second terminal of the eleventh transistor 41 is electricallyconnected to the second output terminal 27, and a gate electrode of theeleventh transistor 41 is electrically connected to the gate electrodeof the second transistor 32 and the gate electrode of the fourthtransistor 34.

In FIG. 26C, a connection point of the gate electrode of the thirdtransistor 33, the gate electrode of the tenth transistor 40, and thesecond terminal of the ninth transistor 39 is referred to as a node NA.In addition, a connection point of the gate electrode of the secondtransistor 32, the gate electrode of the fourth transistor 34, thesecond terminal of the fifth transistor 35, the second terminal of thesixth transistor 36, the first terminal of the eighth transistor 38, andthe gate electrode of the eleventh transistor 41 is referred to as anode NB.

In the case where the pulse output circuit in FIG. 26C is the firstpulse output circuit 10_1, the first clock signal CK1 is input to thefirst input terminal 21, the second clock signal CK2 is input to thesecond input terminal 22, the third clock signal CK3 is input to thethird input terminal 23, the start pulse SP is input to the fourth inputterminal 24, a subsequent-stage signal OUT(3) is input to the fifthinput terminal 25, the first output signal OUT(1)(SR) is output from thefirst output terminal 26, and the second output signal OUT(1) is outputfrom the second output terminal 27.

FIG. 27 is a timing chart of a shift register including the plurality ofpulse output circuits illustrated in FIG. 26C. In the case where theshift register is a gate line driver circuit, a period 61 in FIG. 27 isa vertical retrace period and a period 62 is a gate selection period.

The procedure of supplying, or stopping the supply of, a potential toeach wiring of the driver circuit portion during the operations todisplay a still image and a moving image, or the operation to rewrite avoltage to be applied to a driving TFT of an EL element in a pixelportion (hereinafter the operation is also referred to as refreshoperation), in the driver circuit manufactured using a plurality ofn-channel transistors, which is given as an example in FIGS. 26A to 26Cand FIG. 27, will be described with reference to FIG. 28. FIG. 28illustrates changes in potentials, before and after a period T1, of awiring for supplying a high power supply potential (Vdd), a wiring forsupplying a low power supply potential (Vss), a wiring for supplying astart pulse (SP), and wirings for supplying first to fourth clocksignals (CK1 to CK4) to a shift register.

The liquid crystal display device according to this embodiment candisplay a still image and a mobbing image and display a still image byrefresh operation without constantly operating the driver circuitportion. Therefore, as illustrated in FIG. 28, there are a period inwhich control signals such as the high power supply potential (Vdd), thefirst to fourth clock signals (CK1 to CK4), and the start pulse aresupplied to a shift register and a period in which control signals arenot supplied. Note that the period T1 illustrated in FIG. 28 correspondsto the period in which control signals are supplied, in other words, aperiod in which a moving image is displayed and a period in whichrefresh operation is performed. The period T2 illustrated in FIG. 27corresponds to the period in which control signals are not supplied, inother words, a period in which a still image is displayed.

In FIG. 28, a period in which the high power supply potential (Vdd) issupplied is provided not only in the period T1 but also in part of theperiod T2. In addition, in FIG. 28, a period in which the first tofourth clock signals (CK1 to CK4) are supplied is provided between thestart of the supply of the high power supply potential (Vdd) and thestop of the supply of the high power potential (Vdd).

Moreover, as illustrated in FIG. 28, the first to fourth clock signals(CK1 to CK4) may be set so as to start to oscillate at a constantfrequency after being set to a high potential once before the period T1begins and stop oscillating after being set to a low potential after theperiod T1 ends.

As described above, in the display device according to this embodiment,the supply of control signals such as the high power supply potential(Vdd), the first to fourth clock signals (CK1 to CK4), and the startpulse to the shift register is stopped in the period T2. Then, in theperiod in which the supply of control signals is stopped, whether eachtransistor is turned on or turned off is controlled and the output of apulse signal from the shift register is also stopped. Therefore, powerconsumption of the shift register and power consumption of the pixelportion which is driven by the shift register can be reduced.

Note that the above-described refresh operation needs to be performedregularly because there is a possibility that the quality of a displayedstill image may deteriorate. In the display device according to thisembodiment, the above-described transistor including an oxidesemiconductor is employed as a switching element for controlling avoltage to be applied to a driving TFT of an EL element in each pixel.Thus, off-state current can be drastically decreased, and a change involtage to be applied to the driver TFT of the EL element in each pixelcan be reduced. In other words, even when the period in which theoperation of the shift register is stopped is long due to display of astill image, the deterioration of image quality can be suppressed. Forexample, even when the period is 3 minutes long, the quality of adisplayed still image can be maintained. For example, if a displaydevice in which rewrite is performed 60 times per second and a displaydevice in which refresh operation is performed once in 3 minutes arecompared with each other, power consumption can be reduced toapproximately 1/10000.

Note that the stop of the supply of the high power supply potential(Vdd) is to set a potential equal to the low power supply potential(Vss) as illustrated in FIG. 28. Note that the stop of the supply of thehigh power supply potential (Vdd) may be to set the potential of awiring, to which the high power supply potential is supplied, in afloating state.

Note that when the potential of the wiring to which the high powersupply potential (Vdd) is supplied is increased, which means that thepotential is increased from the low power supply potential (Vss) to thehigh power supply potential (Vdd) before the period T1, it is preferablethat the potential of the wiring is controlled so as to changegradually. If the gradient of the change in potential of the wiring issteep, there is a possibility that the change in potential may becomenoise and an irregular pulse may be output from the shift register. Inthe case where the shift register is included in a gate line drivercircuit, the irregular pulse serves as a signal for turning on atransistor. Thus, there is a possibility that a voltage to be applied toa driver TFT of an EL element might be changed by the irregular pulseand the quality of a still image might be changed. Therefore, it ispreferable that the change in potential of the wiring be controlled asdescribed above. In view of the above content, FIG. 28 illustrates anexample in which a rise in signal to the high power supply potential(Vdd) is more gradual than a fall. In particular, in the display deviceaccording to this embodiment, when a still image is displayed in thepixel portion, the stop of the supply, and the resupply, of the highpower supply potential (Vdd) to the shift register are performed asappropriate. In other words, in the case where a change in potential ofthe wiring for supplying the high power supply potential (Vdd) adverselyaffects the pixel portion as noise, the noise directly leads todeterioration of a display image. Therefore, it is important to controlthe display device according to this embodiment so as to prevent achange in potential (particularly, an increase in potential) of thewiring from entering the pixel portion as noise.

This application is based on Japanese Patent Application serial no.2009-242757 filed with Japan Patent Office on Oct. 21, 2009, andJapanese Patent Application no. 2009-278997 filed with Japan PatentOffice on Dec. 8, 2009, the entire contents of which are herebyincorporated by reference.

The invention claimed is:
 1. A display device comprising: a drivingtransistor comprising: a first gate electrode; a second gate electrodeover the first gate electrode; a first oxide semiconductor layer betweenthe first gate electrode and the second gate electrode; a first sourceelectrode over and electrically connected to the first oxidesemiconductor layer; and a first drain electrode over and electricallyconnected to the first oxide semiconductor layer; a switching transistorcomprising: a second oxide semiconductor layer; a third gate electrodeover the second oxide semiconductor layer; a second source electrodeover and electrically connected to the second oxide semiconductor layer;and a second drain electrode over and electrically connected to thesecond oxide semiconductor layer; and an organic EL element, wherein theswitching transistor does not comprise a gate electrode under the secondoxide semiconductor layer, wherein the organic EL element comprises afirst electrode, a second electrode over the first electrode, and alayer comprising an organic compound between the first electrode and thesecond electrode, wherein one of the second source electrode and thesecond drain electrode is electrically connected to a signal line,wherein the other of the second source electrode and the second drainelectrode is electrically connected to one of the first gate electrodeand the second gate electrode of the driving transistor, wherein one ofthe first source electrode and the first drain electrode is electricallyconnected to the organic EL element, wherein each of the first oxidesemiconductor layer and the second oxide semiconductor layer comprisesindium, gallium, and zinc, wherein a length of the first gate electrodein a channel length direction is larger than a length of the second gateelectrode in the channel length direction, and wherein each of the firstsource electrode and the first drain electrode overlaps with the firstgate electrode.
 2. The display device according to claim 1, furthercomprising a color filter under the first electrode of the organic ELelement.
 3. The display device according to claim 1, wherein the organicEL element is configured to emit white light.
 4. The display deviceaccording to claim 1, wherein a potential of the first gate electrode isdifferent from a potential of the second gate electrode.
 5. The displaydevice according to claim 1, wherein an off-state current of theswitching transistor per a channel width of 1 μm is less than or equalto 1×10⁻¹⁶ A/μm.
 6. The display device according to claim 1, wherein thefirst oxide semiconductor layer and the second oxide semiconductor layereach have a hydrogen concentration of less than or equal to 5×10¹⁹ /cm³.7. The display device according to claim 1, wherein each of a carrierconcentration of the first oxide semiconductor layer and a carrierconcentration of the second oxide semiconductor layer is less than1×10¹⁴ /cm³.
 8. An electronic device comprising the display deviceaccording to claim 1, wherein the electronic device is any of a portablegame machine, a digital camera, a television receiver, a computer, amobile phone, and an e-book reader.
 9. A display device comprising: adriving transistor comprising: a first gate electrode; a second gateelectrode over the first gate electrode; a first oxide semiconductorlayer between the first gate electrode and the second gate electrode; afirst source electrode over and electrically connected to the firstoxide semiconductor layer; and a first drain electrode over andelectrically connected to the first oxide semiconductor layer; aswitching transistor comprising: a second oxide semiconductor layer; athird gate electrode over the second oxide semiconductor layer; a secondsource electrode over and electrically connected to the second oxidesemiconductor layer; and a second drain electrode over and electricallyconnected to the second oxide semiconductor layer; and an organic ELelement, wherein the switching transistor does not comprise a gateelectrode under the second oxide semiconductor layer, wherein theorganic EL element comprises a first electrode, a second electrode overthe first electrode, and a layer comprising an organic compound betweenthe first electrode and the second electrode, wherein one of the secondsource electrode and the second drain electrode is electricallyconnected to a signal line, wherein the other of the second sourceelectrode and the second drain electrode is electrically connected toone of the first gate electrode and the second gate electrode of thedriving transistor, wherein one of the first source electrode and thefirst drain electrode is electrically connected to the organic ELelement, wherein the organic EL element is provided over the drivingtransistor, wherein each of the first oxide semiconductor layer and thesecond oxide semiconductor layer comprises indium, gallium, and zinc,wherein a length of the first gate electrode in a channel lengthdirection is larger than a length of the second gate electrode in thechannel length direction, and wherein each of the first source electrodeand the first drain electrode overlaps with the first gate electrode.10. The display device according to claim 9, further comprising a colorfilter under the first electrode of the organic EL element.
 11. Thedisplay device according to claim 9, wherein the organic EL element isconfigured to emit white light.
 12. The display device according toclaim 9, wherein a potential of the first gate electrode is differentfrom a potential of the second gate electrode.
 13. The display deviceaccording to claim 9, wherein an off-state current of the switchingtransistor per a channel width of 1 μm is less than or equal to 1×10⁻¹⁶A/μm.
 14. The display device according to claim 9, wherein the firstoxide semiconductor layer and the second oxide semiconductor layer eachhave a hydrogen concentration of less than or equal to 5×10¹⁹ /cm³. 15.The display device according to claim 9, wherein each of a carrierconcentration of the first oxide semiconductor layer and a carrierconcentration of the second oxide semiconductor layer is less than1×10¹⁴ /cm³.
 16. An electronic device comprising the display deviceaccording to claim 9, wherein the electronic device is any of a portablegame machine, a digital camera, a television receiver, a computer, amobile phone, and an e-book reader.
 17. A display device comprising: adriving transistor comprising: a first gate electrode; a second gateelectrode over the first gate electrode; a first oxide semiconductorlayer between the first gate electrode and the second gate electrode;and a first electrode over and electrically connected to the first oxidesemiconductor layer; a switching transistor comprising: a second oxidesemiconductor layer; and a third gate electrode over the second oxidesemiconductor layer; an organic EL element, wherein the switchingtransistor does not comprise a gate electrode under the second oxidesemiconductor layer, wherein the organic EL element comprises a secondelectrode, a third electrode over the second electrode, and a layercomprising an organic compound between the second electrode and thethird electrode, wherein the switching transistor is electricallyconnected to a signal line, wherein one of a source and a drain of theswitching transistor is electrically connected to one of the first gateelectrode and the second gate electrode of the driving transistor,wherein one of a source and a drain of the driving transistor iselectrically connected to the organic EL element, wherein each of thefirst oxide semiconductor layer and the second oxide semiconductor layercomprises indium, gallium, and zinc, wherein a length of the first gateelectrode in a channel length direction is larger than a length of thesecond gate electrode in the channel length direction, and wherein thefirst electrode overlaps with the first gate electrode.
 18. The displaydevice according to claim 17, further comprising a color filter underthe first electrode of the organic EL element.
 19. The display deviceaccording to claim 17, wherein the organic EL element is configured toemit white light.
 20. The display device according to claim 17, whereina potential of the first gate electrode is different from a potential ofthe second gate electrode.
 21. The display device according to claim 17,wherein an off-state current of the switching transistor per a channelwidth of 1 μm is less than or equal to 1×10⁻¹⁶ A/μm.
 22. The displaydevice according to claim 17, wherein the first oxide semiconductorlayer and the second oxide semiconductor layer each have a hydrogenconcentration of less than or equal to 5×10¹⁹ /cm³.
 23. The displaydevice according to claim 17, wherein each of a carrier concentration ofthe first oxide semiconductor layer and a carrier concentration of thesecond oxide semiconductor layer is less than 1×10¹⁴ /cm³.
 24. Anelectronic device comprising the display device according to claim 17,wherein the electronic device is any of a portable game machine, adigital camera, a television receiver, a computer, a mobile phone, andan e-book reader.
 25. A display device comprising: a driving transistorcomprising: a first gate electrode; a second gate electrode over thefirst gate electrode; a first oxide semiconductor layer between thefirst gate electrode and the second gate electrode; and a firstelectrode over and electrically connected to the first oxidesemiconductor layer; a switching transistor comprising: a second oxidesemiconductor layer; and a third gate electrode over the second oxidesemiconductor layer; and an organic EL element, wherein the switchingtransistor does not comprise a gate electrode under the second oxidesemiconductor layer, wherein the organic EL element comprises a secondelectrode, a third electrode over the second electrode, and a layercomprising an organic compound between the second electrode and thethird electrode, wherein the switching transistor is electricallyconnected to a signal line, wherein one of a source and a drain of theswitching transistor is electrically connected to one of the first gateelectrode and the second gate electrode of the driving transistor,wherein one of a source and a drain of the driving transistor iselectrically connected to the organic EL element, wherein each of thefirst oxide semiconductor layer and the second oxide semiconductor layercomprises indium, gallium, and zinc, wherein a length of the first gateelectrode in a channel length direction is larger than a length of thesecond gate electrode in the channel length direction, wherein the firstelectrode overlaps with the first gate electrode, wherein the secondelectrode is directly connected to the first electrode, and wherein thefirst electrode and the second gate electrode comprise the samematerial.
 26. The display device according to claim 25, furthercomprising a color filter under the first electrode of the organic ELelement.
 27. The display device according to claim 25, wherein theorganic EL element is configured to emit white light.
 28. The displaydevice according to claim 25, wherein a potential of the first gateelectrode is different from a potential of the second gate electrode.29. The display device according to claim 25, wherein an off-statecurrent of the switching transistor per a channel width of 1pm is lessthan or equal to 1×10⁻¹⁶ A/μm.
 30. The display device according to claim25, wherein the first oxide semiconductor layer and the second oxidesemiconductor layer each have a hydrogen concentration of less than orequal to 5×10¹⁹/cm³.
 31. The display device according to claim 25,wherein each of a carrier concentration of the first oxide semiconductorlayer and a carrier concentration of the second oxide semiconductorlayer is less than 1×10¹⁴/cm³.
 32. An electronic device comprising thedisplay device according to claim 25, wherein the electronic device isany of a portable game machine, a digital camera, a television receiver,a computer, a mobile phone, and an e-book reader.